Formation and nano-scale characterization of ultrahigh-density nanodot superlattices

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Semiconductor nanostructures have attracted much interest due to their quantum-confinement effects, which can cause changes in material opto-electronic properties. We found that ultra-small Ge nanodots with the size of ~5 nm and ultra-high density of ~10^<12> cm^<-2> grew on Si surfaces covered with ultrathin SiO_2 films (~0.3nm thickness). Deposited Ge atoms on the surfaces reacted with the ultrathin SiO_2 films by the chemical reaction of SiO_2+Ge→SiO↑ + GeO↑ to form Ge nanodot nucleation sites. The following deposited Ge atoms were mainly captured by the nucleation sites due to the larger adsorption energy on the nucleation sites than that on the SiO_2 films. This resulted in Ge nanodot formation with ultra-small and uniform size and with ultra-high density. Ge nanodots had spherical shape which was different from that of the Ge island formed by the Stranski-Krastanov growth. Epitaxial or non-epitaxial Ge nanodots could be formed on the substrates by changing substrate temperatures … More during Ge deposition. The ultrathin SiO_2 technolgy could also been applied to form different kinds of nanostructures such as Si nanodots and β-FeSi_2 nanodots or nanoislands.We investigated electronic properties of individual Ge nanodots by scanning tunneling spectroscopy. With decreasing the dot size, the energy band gap of Ge nanodots increased to ~1.4 eV with the size change from 7 to 2 nm, which could be explained by the carrier quantum-confinement effect in Ge nanodots. We also observed the Coulomb-blockade effect featured by discrete tunneling current fluctuation on Ge nanodots at room temperature. The discrete fluctuation was explained by a single electron trap in a nanodot and a single electron escape into Si substrate. We further investigated the optical properties of the stacked structures in which Ge nanodots were embedded in Si films. Intense photoluminescence and electroluminescence were observed at photon energies around 0.8 eV (~1.5 μm) from the structures after high-temperature annealing. The photon energies around 0.8 eV are widely used for the optical communication.We also developed cathode-luminescence and electroluminescence systems using scanning tunneling microscopy to investigate optical properties of an individual nanodot. Less