Nanoscale Quantum Physics Group

Research - Past Findings

Growth of C₆₀ on Ge and Si surfaces

Growth of Ge and Si clusters on C₆₀

The overall goal of this project is to develop C₆₀ overlayers as a substrate on which to fabricate nano structures of semiconductors or metals. The C₆₀ layer has a band gap close to 2eV, which will prevent the carrier in the nano structure from leaking out to the substrate. The C ₆₀ molecule is also stable and relatively inert. Some recent findings resulting from our investigations of C ₆₀ thin film formation are summarized below.

A new mechanism by which the ordering of molecules on anisotropic surfaces takes place has been determined. Because the interaction potential between the C₆₀ molecule and the surface is highly anisotropic, it only defines the ordering of the adsorbate in one principal axis while the balance between the repulsive and attractive forces among the molecules determines ordering along the other axis.

We have shown that from STM images it is possible to infer some of the events that result in energy dissipation of the thermal molecules prior to being bound onto the surface. This realization came from the observation that C₆₀ molecules prefer to form a pair occupying certain sites on the surface, which could only be explained by an inelastic collision and sticking process, not diffusive processes.

The microscopic origin that leads to the rotational epitaxy for multi-layer C₆₀ on Ge and Si surfaces has been determined. Such rotation occurs gradually throughout a number of layers, probably to diffuse strain. This coherent rotational epitaxy has not be reported before.

A novel bi-step flow growth mode for epitaxial growth of large molecules on atomic substrates has been observed in the C₆₀/Ge and C₆₀/GeAs systems. This occurs when the size of adsorbate molecules is much larger than the monoatomic step height. This has broad implications for epitaxial growth of organic molecules or patterning passivated quantum dots on atomic substrates, an emerging technique for fabrication of hybrid devices.

Isolated Si and Ge clusters have been grown on ordered C₆₀ overlayers on Si and Ge substrates. Si and Ge clusters nucleate in the interstices of the C₆₀ layer, and reach a typical lateral size of 10 Å before coalescing. Nontetrahedral coordination of Si atoms in the clusters was detected from Auger spectra results. Room temperature tunneling spectroscopy of isolated clusters reveals an enlarged band gap consistent with the effect of quantum confinement, and a quantized conductance attributable to the Coulomb blockade phenonmena in a tip/cluster/substrate double junction barrier. Our preliminary experiments show that metallic dots can also be fabricated using this technique.