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.     

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     

Double diffraction spots in RHEED patterns from clean Ge(111) and Si(001) surfaces

In RHEED patterns from clean Ge(111) and Si(001) surfaces, extra diffraction spots have been observed with superlattice reflection spots due to Ge(111) 2 × 8 and Si(001) 2 × 1 surface structures. The extra spots have not been found out in many previous LEED and RHEED patterns of clean Ge(111) and Si(001) surfaces. When the Ge(111) and Si(001) samples were rotated about an axis normal to the surfaces so as to vary the incident direction of the primary electron beam, the intensity of the extra spots showed a remarkable dependence upon the incident direction and they became invisible in some incident directions, in spite of the experimental condition that an Ewald sphere intersected reciprocal lattice rods of the extra spots. In this study, the extra spots are understood as forbidden reflection spots resulting from double diffraction of superlattice reflections of the surface structures, and the remarkable dependence of their intensity upon the incident direction is explained in terms of excitation of the surface wave of the superlattice reflections. These results suggest that the intensity of diffraction spots in RHEED patterns may be greatly influenced by the surface wave excitation of fundamental and superlattice reflections.     

C60 growth on Si(100), GaSe(0001) and GeS(001)

C₆₀ films have been grown in ultra high vacuum on various crystalline substrates and the structure of the films has been investigated by low energy electron diffraction (LEED) and high resolution electron energy loss spectroscopy (HREELS). The C₆₀ films form randomly oriented nanocrystals on Si(100), mesoscopic polycrystals on GaSe(0001) and microscopic single crystals on GeS(001). The vibrational structure of the C₆₀/substrate interfaces is analyzed in detail by HREELS carried out in the dipole and impact scattering regimes. It is shown that the epitaxy of C₆₀ on GeS(001) is induced by the weak van der Waals bonding and the peculiar corrugation of the substrate surface.     

Structural state of Ge/Si heterosystems with (001), (111), and (7 7 10) interfaces

It is shown for (111) and (001) interfaces that at an identical degree of strain relaxation in semi-conductor epitaxial films, the ratio of distances D between neighboring dislocations is D ₍₁₁₁₎/D ₍₀₀₁₎ = 1.5. This allows us to establish that dislocation interface (7 7 10) contains partial 90° Shockley dislocations lying in three directions of 〈110〉.     

Si(001)表面In量子线的第一原理研究

利用第一原理理论研究了金属In在Si(001)表面吸附的原子结构.结果表明,In原子的吸附不破坏衬底Si的二聚体化学键.在低覆盖率时,In原子在Si(001)衬底上形成有序量子线,取向沿衬底Si的二聚体化学键方向.计算结果显示相邻In线之间不存在排斥作用.     

硅(001)图形衬底上锗硅纳米线的定位生长

纳米线的定位生长是实现纳米线量子器件寻址和集成的前提.结合自上而下的纳米加工和自下而上的自组装技术,通过分子束外延生长方法,在具有周期性凹槽结构的硅(001)图形衬底上首先低温生长硅锗薄膜然后升温退火,实现了有序锗硅纳米线在凹槽中的定位生长,锗硅纳米线的表面晶面为(105)晶面.详细研究了退火温度、硅锗的比例及图形周期对纳米线形成与否,以及纳米线尺寸的影响.     

Structural transformation of Ge dimers on Ge（001） surfaces induced by bias voltage

Scanning tunnelling microscopy is utilized to investigate the local bias voltage tunnelling dependent transformation between （2×1） and c（4×2） structures on Ge（001） surfaces, which is reversibly observed at room temperature and a critical bias voltage of -0.80 V. Similar transformation is also found on an epitaxial Ce islands but at a slightly different critical bias voltage of -1.00V. It is found that the interaction between the topmost atoms on the STM tip and the atoms of the dimers, and the pinning effect induced by Sb atoms, the nacancies or the epitaxial clusters, can drive the structural transformation at the critical bias voltage.     

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).     

Surface diffusion of Si,Ge and C adatoms on Si （001） substrate studied the molecular dynamics simulation

Depositions of Si, Ge and C atoms onto a preliminary Si (001) substrate at different temperatures are investigated by using the molecular dynamics method. The mechanism of atomic self-assembling occurring locally on the flat terraces between steps is suggested. Diffusion and arrangement patterns of adatoms at different temperatures are observed. At 900 K, the deposited atoms are more likely to form dimers in the perpendicular [110] direction due to the more favourable movement along the perpendicular [110] direction. C adatoms are more likely to break or reconstruct the dimers on the substrate surface and have larger diffusion distances than Ge and Si adatoms. Exchange between C adatoms and substrate atoms are obvious and the epitaxial thickness is small. Total potential energies of adatoms and substrate atoms involved in the simulation cell are computed. When a newly arrived adatom reaches the stable position, the potential energy of the system will decrease and the curves turns into a ladder-like shape. It is found that C adatoms can lead to more reduction of the system energy and the potential energy of the system will increase as temperature increases.     

Formation mechanism of one-dimensional Si islands on a H/Si(001) surface

The formation mechanism of one-dimensional Si islands on a H/Si(001)-(2x1) surface is studied using scanning tunneling microscopy/spectroscopy and first-principles calculations. We observed that one-dimensional islands that are made from dimer chains are formed at the initial growth stages similar to the bare Si(001) surface. It is found that the number of odd-numbered dimer chains is larger than that of even-numbered dimer chains. We propose the growth processes of the two types of growth edges to explain the observation.