Endoepitaxial growth of hexagonal-Fe13Ge8 islands on Cubic-Ge(001)

The growth and shape evolution of epitaxial Fe₁₃Ge₈ (hexagonal lattice) islands on single crystal Ge(001) (cubic lattice) substrate was observed in real time using an in situ ultra-high vacuum transmission electron microscope (TEM). Post-deposition high-resolution TEM in conjunction with stereographic projection enabled the identification of the interface structure between the Fe₁₃Ge₈ islands and the Ge substrate. Only one low-energy coherent interface formed via Fe₁₃Ge₈ islands growing into the substrate along the inclined Ge(11?1) plane. This indicates that minimization of net interfacial energy is the driving force for hexagonal Fe₁₃Ge₈ islands formation on Ge(001).     

Dome-to-pyramid transition induced by alloying of Ge islands on Si(001)

Received: 19 October 1998/Accepted: 19 October 1998     

Tunnelling spectroscopy on silver islands and large deposited silver clusters on Ge(001)

Morphology and electronic properties of silver islands and deposited silver clusters on Ge(001) have been studied using scanning tunnelling microscopy (STM) and spectroscopy (STS) at low temperatures. Already the clean surface bears an interesting electronic structure, which is obvious from the STS. The tunnelling spectra exhibit strong peaks associated with dangling bond-derived surface states and an antibonding σ-state of the surface dimer. For silver islands of only few monolayers in height, complex spectra are interpreted to be dominated by metal–semiconductor interface states. These islands show energy gaps which are not observed for larger ones beyond 1 nm in height. Spectra of the larger islands contain a series of distinct peaks originating from lateral and three-dimensional electron confinement, respectively. Silver clusters – preformed in the gas phase using a cluster source – have been fabricated, size selected and deposited onto germanium(001). In tunnelling spectra dips at the Fermi level are accompanied by two maxima. These characteristics seem to be almost independent from the cluster size. Additional weak structures are found at higher bias voltages, which are understood in terms of quantized states. PACS 36.40.Cg; 61.46.+w; 73.20.At     

Towards quantitative understanding of formation and stability of Ge hut islands on Si(001)

We analyze Ge hut island formation on Si(001), using first-principles calculations of energies, stresses, and their strain dependence of Ge/Si(105) and Ge/Si(001) surfaces combined with continuum modeling. We give a quantitative assessment on strain stabilization of Ge(105) facets, estimate the critical size for hut nucleation or formation, and evaluate the magnitude of surface stress discontinuity at the island's edge and its effect on island stability.     

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.     

Cobalt induced nanocrystals on Ge(001)

The deposition of several monolayers of cobalt on germanium (001) substrates results in the formation of two types of clusters: flat-topped and peaked nanocrystals. Scanning tunneling spectroscopy and helium ion microscopy measurements reveal that these nanocrystals contain cobalt. The shape evolution of the flat-topped and peaked nanocrystals as a function of their size is investigated with scanning tunneling microscopy. For small sizes the nanocrystals are compact. Beyond a critical size, however, the peaked nanocrystals exhibit an elongated shape, whilst the flat-topped nanocrystals remain compact. The shape transition of the peaked nanocrystals is driven by a competition between boundary and strain energies. For small sizes the boundary energy is the dominant term leading to a minimization of the peaked nanocrystal's perimeter, whereas at larger sizes the strain energy wins resulting in a maximization of the perimeter. On the top facet of the flat-topped nanocrystals one-dimensional structures are observed that are comprised of small square shaped units of about 1 nm². Time-resolved scanning tunneling microscopy measurements reveal that these square shaped units are dynamic at room temperature.     

Formation of coherent Ge shallow dome islands on Si(001) by ultra-high-vacuum ion beam sputter deposition

Low-temperature (∼ 400 °C) metal-induced crystallization of hydrogenated amorphous silicon–germanium thin films using Au solution has been investigated by X-ray diffraction, Raman spectra, scanning electron microscopy and atomic force microscopy. It was shown that Au solution significantly promotes the crystallization of the films at low temperatures. The effects of annealing temperature, Ge fraction in the films and the concentration of Au solution on the structure and morphology of the films were analyzed. An increase in crystallinity was observed on increasing the annealing temperature and concentration of Au solution. High-frequency shifts of Ge-Ge and Si-Ge modes and a low-frequency shift of the Si-Si mode were found on increasing the Ge fraction. Compared with Au-induced crystallization, the Au solution induced crystallization tends to give large crystal grains. PACS 81.10.-h; 81.10.Aj; 81.10.Jt; 81.05.Gc; 81.15.Gh     

Valence surface electronic states on Ge(001)

The optical, electrical, and chemical properties of semiconductor surfaces are largely determined by their electronic states close to the Fermi level (E{F}). We use scanning tunneling microscopy and density functional theory to clarify the fundamental nature of the ground state Ge(001) electronic structure near E{F}, and resolve previously contradictory photoemission and tunneling spectroscopy data. The highest energy occupied surface states were found to be exclusively back bond states, in contrast to the Si(001) surface, where dangling bond states also lie at the top of the valence band.     

Dynamics of Au-induced nanowires on Ge(001)

We have studied the dynamics of Au-induced nanowires on Ge(001) using scanning tunnelling microscopy. The ridges of these nanowires consist of buckled dimers that have their dimer bond aligned in a direction perpendicular to the nanowire direction. Neighbouring dimers have a preference to buckle in an opposite direction, leading to rows with a zigzag appearance. Dimers located at anti-phase boundaries continuously flip back and forth between their two buckled configurations. This process is thermally induced and occurs at an average frequency of 25 Hz at room temperature.     

Ripening of single-layer InGaAs islands on GaAs（001）

The present paper discusses our investigation of InGaAs surface morphology annealed for different lengths of time.After annealing for 15 min,the ripening of InGaAs islands is completed.The real space scanning tunneling microscopy(STM) images show the evolution of InGaAs surface morphology.A half-terrace diffusion theoretical model based on thermodynamic theory is proposed to estimate the annealing time for obtaining flat morphology.The annealing time calculated by the proposed theory is in agreement with the experimental results.     

Pt-chain induced formation of Ge nanowires on the Ge(001) surface

A new reconstructed Pt/Ge(001)–4 × 2 surface structure of 0.25 ML Pt deposition is suggested based on density functional theory. The Ge dimers form nanowire arrays on a Pt-chain modified Ge(001) surface in which the chain is located between the two quasi-dimer rows and below the Ge nanowire. The simulated scanning tunneling microscope (STM) images of the surface are in excellent agreement with the previously observed STM features and sample bias dependence. It is the nanowire Ge dimers and not the Pt atoms that contribute to the STM images for occupied states at high sample biases, contrary to what has always been assumed in experiments. The surface bands of the Pt chain and quasi-dimer rows exhibit quasi-one-dimensional metallic behavior in the direction of the nanowire. When changing from the 4 × 2 to the 4 × 4 structure, there are likely pseudogaps opened at the new surface Brillouin zone boundary, which simultaneously reduce the metallicity. This may be related to the Peierls instability. The interaction between the Pt chain and the quasi-dimer row, as well as the inter-quasi-dimer row interaction, is of essential importance for stabilization.     

Surface stress anisotropy of Ge(001)

By analyzing the equilibrium shape of vacancy islands on the Ge(001) surface we have determined the surface stress anisotropy, i.e., the difference between the compressive stress component along the substrate dimer rows and the tensile stress component perpendicular to the substrate dimer rows. In order to extract the surface stress anisotropy we have used a model recently put forward by Li et al. [Phys. Rev. Lett. 85, 1922 (2000)]. The surface stress anisotropy of the clean Ge(001) surface is found to be 80+/-30 meV/A(2). This value is comparable to the surface stress anisotropy of the closely related Si(001) surface.     

Influence of the germanium deposition rate on the growth and Photoluminescence of Ge(Si)/Si(001) self-assembled islands

The growth and photoluminescence of Ge(Si)/Si(001) self-assembled islands are investigated over a wide range of germanium deposition rates v_Ge = 0.1–0.75 Å/s at a constant growth temperature T _g = 600°C. Examination of the surface of the grown structures with an atomic force microscope revealed that, for all the germanium deposition rates used in the experiments, the dominant island species are dome islands. It is found that an increase in the deposition rate v_Ge leads to a decrease in the lateral size of the self-assembled islands and an increase in their surface density. The decrease in the lateral size is associated both with the increase in the germanium content in the self-assembled islands and with the increase in the fraction of the surface occupied by these islands. The observed shift in the position of the photoluminescence peak toward the low-energy range is also explained by the increase in the germanium content in the islands with an increase in the deposition rate v_Ge.     

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     

Diffusion driven concerted motion of surface atoms: Ge on Ge(001)

The diffusion of Ge dimers on the Ge(001) surface has been studied with scanning tunneling microscopy. We have identified three different diffusion pathways for the dimers: diffusion of on-top dimers over the substrate rows, diffusion across the substrate rows, and diffusion of dimers in the trough. We report on a heretofore unknown phenomenon, namely, diffusion driven concerted motion of substrate atoms. This concerted motion is a direct consequence of the rearrangement of substrate atoms in the proximity of the trough dimer adsorption site.     

Instability of one-dimensional dangling-bond wires on H-passivated C(001), Si(001), and Ge(001) surfaces

We investigate the instability of one-dimensional dangling-bond (DB) wires fabricated on the H-terminated C(001), Si(001), and Ge(001) surfaces by using density-functional theory calculations. The three DB wires are found to show drastically different couplings between charge, spin, and lattice degrees of freedom, resulting in an insulating ground state. The C DB wire has an antiferromagnetic spin coupling between unpaired DB electrons, caused by strong electron–electron interactions, whereas the Ge DB wire has a strong charge-lattice coupling, yielding a Peierls-like lattice distortion. For the Si DB wire, the antiferromagnetic spin ordering and the Peierls instability are highly competing with each other. The physical origin of such disparate features in the three DB wires can be traced to the different degree of localization of 2p, 3p, and 4p DB orbitals.