H.H. Weitering

Epitaxial ferromagnetic Mn5Ge3 on Ge(111)

Ferromagnetic Mn5Ge3 thin films were grown on Ge(111) with solid-phase epitaxy. The epitaxial relationship between the alloy film and substrate is Mn5Ge3(001)//Ge(111) with [100]Mn5Ge3//[110]Ge. The alloy films exhibit metallic conductivity and strong ferromagnetism up to the Curie temperature, TC=296 K. These epitaxial alloy films are promising candidates for germanium-based spintronics.

Ferromagnetic percolation in MnxGe1−x dilute magnetic semiconductor

We have studied the magnetic and magnetotransport properties of Mn-doped Ge grown by molecular-beam epitaxy. This group-IV dilute ferromagnetic semiconductor exhibits two magnetic transitions. An upper critical temperature TC* (∼112K for x∼0.05) is evident from the extrapolated Curie–Weiss susceptibility and from the Arrott plot analysis of anomalous Hall effect data. The existence of a lower critical temperature TC (∼12K for x∼0.05) is established from ac susceptibility and magnetotransport data. The data are fully compatible with the existence of bound magnetic polarons or clusters below TC* which percolate at TC⪡TC*.

New barium-induced surface reconstructions on Si(111)

Abstract The growth of Ba on Si(111)7 × 7 at room temperature proceeds in a layer-by-layer fashion up to at least two full layers. Upon subsequent annealing, four new sub-monolayer surface reconstructions can be identified. The (3 × 1), (5 × 1) and (2 × 8) reconstructions are formed at absolute coverages not larger than 0.35, 0.50 and 0.65 monolayer (ML), respectively. A (√3 × √3)R30° reconstruction co-exists with each of these phases and possibly comprises a pseudo-centered orthorhombic BaSi 2 (100) layer. Epitaxial BaSi 2 is formed under supersaturation conditions at substrate temperatures near 600°C.

Electronic properties of the AgGe(111) interface

Abstract When silver is deposited onto Ge(111)c(2 × 8), the interface may undergo a (3 × 1), (4 × 4), (√3 × √3)R30 and eventually a (6 × 6) reconstruction. By using a scanning tunneling microscope (STM) in the current-imaging tunneling spectroscopy (CITS) mode, we obtained important new information on the electronic properties of these coexisting reconstructions. The c(2 × 8), (3 × 1) and (4 × 4) reconstructions are non-metallic with tunneling gaps of 0.45, 0.85 and 0.85 eV, respectively, while the (√3 × √3)R30° reconstruction is manifestly metallic. Electron energy-loss spectroscopy (EELS) data reveal Δ q | = 0 excitation gaps which match the tunneling gaps from CITS. In addition, EELS indicates that increasing the Ag density in the metallic (√3 × √3)R30° phase initiates a gradual metal-non-metal transition. This transition is completed with the formation of a high-density (6 × 6) reconstruction with a band gap of 0.18 eV. We propose that the (4 × 4), (√3 × √3)R30° and (6 × 6) structures have a Ge trimer unit in common.

Scanning tunneling microscopy study of the metal-induced Si(111)3 × 1 reconstruction: evidence for dimerized chain formation

Abstract Scanning tunneling microscope (STM) images of the Si(111)3 × 1-Ag interface exhibit a strong bias dependence. A clear dimerization of the Si surface atoms is evident from the 1 V empty state images. The imaged electronic states exhibit either a 3 × 1 or a 6 × 1 periodicity. These results are interpreted on the basis of the recently proposed dimerized chain or conjugated chain model. It is demonstrated that the dimer orientation in every second dimer row of the 3 × 1 reconstruction can be changed using a STM tip, thereby producing a 6 × 1 symmetry. This phenomenon could be similar to the well-known soliton excitations in doped conjugated polymers.

Ga-induced atom wire formation and passivation of stepped Si(112)

We present an in-depth analysis of the atomic and electronic structure of the quasi-one-dimensional 1D surface reconstruction of Ga on Si112 based on scanning tunneling microscopy and spectroscopy STM and STS, Rutherford-backscattering spectrometry RBS, and density functional theory DFT calculations. A new structural model of the Si1126 1-Ga surface is inferred. It consists of Ga zigzag chains that are intersected by quasiperiodic vacancy lines or misfit dislocations. The experimentally observed meandering of the vacancy lines is caused by the coexistence of competing 6 1 and 5 1 unit cells and by the orientational disorder of symmetry breaking Si- Ga dimers inside the vacancy lines. The Ga atoms are fully coordinated, and the surface is chemically passivated. STS data reveal a semiconducting surface and show excellent agreement with calculated local density of states LDOS and STS curves. The energy gain obtained by fully passivating the surface calls the idea of step-edge decoration as a viable growth method toward 1D metallic structures into question.

Electron localization and the nonmetal-to-metal transition at ultrathin alkali-metal/Si(111) interfaces

Abstract The K/Si(111)7 × 7 and K/Si(111)(√3 × √3)R30°-B interfaces are nonmetallic at room temperature saturation coverage. In contrast, the Cs/Si(111)7 × 7 interface metallizes below saturation coverage. Possible mechanisms of electron localization in pre-metallized films will be discussed. We conclude that electron correlation effects are important in determining the electronic properties of these interfaces.

Isolation of a metallic Si(111)7×7 surface reconstruction via separation by implanted oxygen

High-quality Si(111)7×7 surface reconstructions have been observed on (111)-oriented Si/SiO2/Si substrates, prepared via separation by implantation of oxygen, or “SIMOX,” with top layer thicknesses as small as 220 A. Scanning tunneling microscopy and spectroscopy data indicate that the electrically and physically isolated top layer is electrically conducting, in contrast to that of (100) SIMOX material, which accumulates charge under typical imaging conditions. We speculate that the 7×7 reconstruction on (111) SIMOX material is an efficient conduction channel, allowing atomic resolution imaging of the isolated Si top layer.

Quantum Size Effects in the Growth and Properties of Ultrathin Metal Films, Alloys, and Related Low-Dimensional Structures

This chapter addresses the quantum mechanical nature of the formation, stability, and properties of ultrathin metal films, metallic alloys, and related low-dimensional structures, with Pb as a primary elemental example. The emphasis is on the contribution to the overall energetics from the electronic degrees of freedom of the low-dimensional systems. As a metal film reduces its thickness, the competition between quantum confinement, charge spilling, and Friedel oscillations, all of electronic origin, can dictate whether an atomically smooth film is marginally, critically, or magically stable or unstable against roughening during the growth of such metal films. The “electronic growth” mode as emphasized here serves as an intriguing addition to the three well-established classic modes of crystal growth. In exploring electronic growth, Pb(111) films represent a particularly compelling example, not only because their stability exhibits unusually strong quantum oscillations but also because their physical and chemical properties can be tuned with great precision by controlling the film thickness or the chemical composition. Recent advances and the perpectives in this active area of film growth will be reviewed, with results from both theoretical and experimental studies.