No abstract

Heteroepitaxial Ge͞Si(001) growth has been investigated using in situ scanning tunneling microscopy. While at 620 K the epitaxial strain is relieved by formation of three-dimensional islands (so-called "hut" clusters), at 690 K the strain is first relieved by hut pits, having the cluster shapes but with their apex pointing down. Although predicted theoretically to have lower energy than clusters, hut pits have never been observed individually before. Details of cluster and pit nucleation are also presented for the first time.

TaO(x)-based memristors have recently demonstrated both subnanosecond resistance switching speeds and very high write/erase switching endurance. Here we show that the physical state variable that enables these properties is the oxygen concentration in a conduction channel, based on the measurement of the thermal coefficient of resistance of different TaO(x) memristor states and a set of reference Ta-O films of known composition. The continuous electrical tunability of the oxygen concentration in the channel, with a resolution of a few percent, was demonstrated by controlling the write currents with a one transistor-one memristor (1T1M) circuit. This study demonstrates that solid-state chemical kinetics is important for the determination of the electrical characteristics of this relatively new class of device.

Self-assembled α-FeSi(2) nanoislands were formed using solid-phase epitaxy of low (~1.2 ML) and high (~21 ML) Fe coverages onto vicinal Si(111) surfaces followed by thermal annealing. At a resulting low Fe-covered Si(111) surface, we observed in situ, by real-time scanning tunneling microscopy and surface electron diffraction, the entire sequence of Fe-silicide formation and transformation from the initially two-dimensional (2 × 2)-reconstructed layer at 300 °C into (2 × 2)-reconstructed nanoislands decorating the vicinal step-bunch edges in a self-ordered fashion at higher temperatures. In contrast, the silicide nanoislands at a high Fe-covered surface were noticeably larger, more three-dimensional, and randomly distributed all over the surface. Ex situ x-ray photoelectron spectroscopy and high-resolution transmission electron microscopy indicated the formation of an α-FeSi(2) island phase, in an α-FeSi(2){112} // Si{111} orientation. Superconducting quantum interference device magnetometry showed considerable superparamagnetism, with ~1.9 μ(B)/Fe atom at 4 K for the low Fe-coverage, indicating stronger ferromagnetic coupling of individual magnetic moments, as compared to high Fe-coverage, where the calculated moments were only ~0.8 μ(B)/Fe atom. Such anomalous magnetic behavior, particularly for the low Fe-coverage case, is radically different from the non-magnetic bulk α-FeSi(2) phase, and may open new pathways to high-density magnetic memory storage devices.

Investigation of analog memristive switching of iron oxide nanoparticle assembly between Pt electrodes

No abstract

The growth of Ge three-dimensional coherent clusters on Si͑001͒ during gas source molecular-beam epitaxy and post-deposition anneals has been investigated using in situ elevated-temperature scanning tunneling microscopy. By monitoring the growth of individual so-called ''hut'' clusters, this technique allowed us to separate various factors that may affect the final size distribution of entire cluster ensembles. It has been found that during the course of epitaxy the hut clusters grow by nucleation and growth of deposited material on the cluster facets; however, the low growth rate ͑rϰt 1/n , where 4ϽnϽ5͒, and the large scatter in absolute rate constants indicate diffusion-limited, rather than interface-limited mass transport, although both the facetnucleation step and the wetting-layer defects inhibit the cluster growth. The strain-induced energy barriers at the cluster bases prevent material addition, and thus growth of large clusters, facilitating the domination of growth mechanisms other than Ostwald ripening at temperatures below 700 K, leading to symmetric or positively skewed cluster-size distribution functions. The tendency towards negatively skewed and bimodal distributions at higher temperatures signaled the contribution of the ripening in accord with the Lifshitz-Slyozov-Wagner theory. Raising the temperature above 770 K leads to a gradual replacement of the huts by the ͗110͘-based macroscopic clusters. ͓S0163-1829͑97͒05039-X͔

We characterized the conduction mechanisms in thin sputtered films of three representative binary Me-O (Me = Ta, W, and Nb) systems as a function of oxygen content, by combining in situ chemical state and electronic band structure studies from X-ray photoemission with temperature-dependent transport measurements. Despite certain differences, these amorphous films all displayed Fermi glass behavior following an oxidation-induced transition from metallic to hopping conduction, down to a sub-percolation threshold. The electron localization estimated from the band structure was in good agreement with that from the transport measurements, and the two were used to construct phase diagrams of conduction in the degree of oxidation-conductivity coordinates, which should prove important in the design of resistive switching and other electronic devices.