No abstract

ZnO nanopillars are often grown on various substrates by catalytic growth processes through a vapor-liquid-solid (VLS) mechanism. However, on silicon substrates, even with the catalyzed growth processes, it is still very difficult to obtain highly oriented ZnO nanopillar arrays. In this work, it was found that in most cases the actual growth process of ZnO on Si catalyzed by Au was not of real VLS character. In the initial growth stage, the substrate surface is partially melted and then oxidized into a very thin layer of SiO2. Zn-rich alloys instead of ZnO are first deposited on the SiO2∕Si substrates and form polycrystalline hillocks in an atmosphere with low O2 partial pressure. The difficulty for ZnO to nucleate on SiO2∕Si is another reason preventing ZnO nanopillars from growing epitaxially on the substrates. Defects, steps, and∕or stress on the substrate surfaces may support the nucleation process and thus may influence the initial growth stage and the control of the growth orientation of the pillars. With the help of scratches on Si substrates and by avoiding the formation of hillocks, well-aligned ZnO nanopillars were obtained both on Si (111) and (100) surfaces. Sharp photoluminescence (PL) peaks of bound exciton emissions and a free exciton emission were detected at low temperature. An unreported PL peak at about 3.367eV and in some cases a donor-acceptor-pair transition at about 3.308eV were also found. At higher temperature, a band-acceptor recombination process together with strong LO-phonon replicas occurred. A donor dopant concentration ND of about 5.7×1018cm−3 and an acceptor ionization energy of 126±2meV were evaluated from the spectra.

Using the vapor-liquid-solid (VLS) technique, we have grown well-aligned nanopillars on [112¯0]-sapphire (a-plane) substrates at atmospheric pressure in a horizontal tube furnace employing gold catalyst seeds of different sizes and densities. It was the aim of the present work to find experimental conditions (source and template temperatures, temperature gradients, carrier gas flow, gold cluster size and density) under which controlled catalytic growth of nanopillars takes place. The VLS process is expected to result in a correlation of the zinc oxide (ZnO) pillar diameters with the gold catalyst cluster size. This is indeed found to hold true except for very small gold clusters. A minimum value of the pillar diameter of about 20 nm on a-plane sapphire is obtained in our experiments, which apparently represents a general limit. Structure characterization relies on high-resolution x-ray diffraction, atomic force microscopy, and high-resolution scanning electron microscopy. Electronic characterization is done by Raman and standard luminescence measurements with large area (≈1 mm2) excitation. In particular, we have also studied individual nanopillars by spatially highly resolved cathodoluminescence spectroscopy. We find that luminescence averaged over large areas can be dominated by features which are not typical for well-shaped pillars but come from regions of irregular ZnO growth.

In this paper we present a novel approach to fabricate single-electron devices utilizing different self-organization and self-alignment effects. Silicon quantum dots (QDs) are obtained employing reactive ion etching (RIE) into a silicon-on-insulator (SOI) substrate with a selfassembled etch mask. Electrodes with nanometer separation are fabricated and aligned to the QDs by means of a controlled electromigration process. The tunneling rates of the devices are defined by the native oxide covering the silicon QDs and can be adjusted by self-limiting thermal oxidation. The devices show clear Coulomb blockade behavior as well as Coulomb staircase features. In some samples also a gate influence is present giving rise to Coulomb diamonds in the differential conductance diagram.Mater. Res. Soc. Symp. Proc. Vol. 958