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).     

Surface superstructures of the Pb/Si(001) system

In the present work the Pb/Si(001) system has been studied with LEED (low-energy electron diffraction), AES (Auger electron spectroscopy), and EELS (electron energy loss spectroscopy). Five different surface superstructures, i.e., the 2 × 2, c(4 × 8), 4 × 1, 2 × 1 and c(4 × 4) were observed. Four of them are found for the first time, except for the 2 × 1. Their relationship has been investigated as a function of Pb coverage and annealing temperature. As a result, a complete phase diagram of the system has been determined. Upon annealing at 450°C the 2 × 2 superstructure undergoes an irreversible phase transformation to the c(4 × 8), while the c(4 × 4) reversibly transforms to 2 × 1 at 300°C. Strong influence of oxygen contamination on the surface superstructures has been observed.     

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     

Adsorption of Au and Pt dimers on Ge(001) and Si(001): A first-principles study

Based on first-principles total energy calculations, the adsorption of Au and Pt dimers on Ge(001) and Si(001) surfaces are investigated. We find that the Au dimer on both Ge(001) and Si(001) show a similar result with the most stable configuration C, parallel to the substrate dimer row and located in the trough between the dimer rows, and the most unstable configuration A, parallel to and on the top of the substrate dimer row. On the other hand, Pt dimer on Ge(001) prefer the configuration D, perpendicular to the substrate dimer row and located in the trough between the dimer rows, while Pt dimer on Si(001) prefer both A and D configurations. The different structural stabilities of Au and Pt dimers on Ge(001) and Si(001) surfaces are attributed to the different electronic structures of Au and Pt atoms. These results are discussed with the reported data for III, IV and V group elements on Si(001).     

Electronic Structure of Si1-xⅣx/Si Superlattices on Si （001）

We have preformed systematical ab initio studies of the structural and electronic properties of short-period Si1-xⅣx/Si （x = 0.125, 0.25, 0.5, Ⅳ=Ge, Sn） superlattices （SLs） grown along the [001] direction on bulk Si. The present calculations reveal that the Si0.875Ge0.125/Si, Si0.75Ge0.25/Si and Si0.875Sn0.125/Si are the F-point direct bandgap semiconductors. The technological importance lies in the expectation that the direct gap Si1-xⅣx/Si SLs may be used as components in integrated optoelectronic devices, in conjunction with the already well-established and highly advanced silicon technology.     

Suppressing Effects of Ag Wetting Layer on Surface Conduction of Er Silicide/Si(001) Nanocontacts

Current-voltage electrical characteristics of Er silicide/Si(001) nanocontacts are measured in situ in a scanning tunneling microscopy system. Introduced as a new technique to suppress surface leakage conduction on Si(001),a silver wetting layer is evaporated onto the substrate surface kept at room temperature with Er Si2 nanoislands already existing. The effects of the silver layer on the current-voltage characteristics of nanocontacts are discussed.Our experimental results reveal that the silver layer at coverage of 0.4–0.7 monolayer can suppress effectively the current contribution from the surface conduction path. After the surface leakage path of nanocontacts is obstructed, the ideality factor and the Schottky barrier height are determined using the thermionic emission theory, about 2 and 0.5 eV, respectively. The approach adopted here could shed light on the intrinsic transport properties of metal-semiconductor nanocontacts.     

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.     

Structural phase transition on Si(001) and Ge(001) surfaces

Among a variety of solid surfaces, Si(001) and Ge(001) have been most extensively studied. Although they seem to be rather simple systems, there have been many conflicting arguments about the atomic structure on these surfaces. We first present experimental evidence indicating that the buckled dimer is the basic building block and that the structural phase transition between the low-temperature c(4x2) structure and the high-temperature (2x1) structure is of the order-disorder type. We then review recent theoretical work on this phase transition. The real system is mapped onto a model Ising-spin system and the interaction parameters are derived from total-energy calculations for different arrangements of buckled dimers. The calculated critical temperature agrees reasonably well with the experimental one. It is pointed out that the nature of the phase transition is crucially affected by a small amount of defects on the real surfaces.