Anton Reiser

Conductivity of single ZnO nanorods after Ga implantation in a focused-ion-beam system

ZnO nanorods were implanted with Ga+ ions in a combined scanning-electron-microscope/focused-ion-beam system with doses from 1011to1017cm−2. Electrical resistance measurements performed on single ZnO nanorods yield first an increase of the resistance due to defect formation which lowers the electron mobility. Implantation doses exceeding 1015cm−2 yield a strong decrease of the resistance to values significantly below the resistance before Ga+-ion implantation. Low specific resistivities of about 3×10−3Ωcm are reached without additional annealing treatment after high-dose implantation.

Au-catalyzed growth processes and luminescence properties of ZnO nanopillars on Si

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...

Controlled catalytic growth and characterization of zinc oxide nanopillars on a-plane sapphire

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 do...

Cathodoluminescence study of single zinc oxide nanopillars with high spatial and spectral resolution

Single zinc oxide (ZnO) nanopillars grown by the vapor-liquid-solid process on a-plane sapphire are investigated by spatially resolved cathodoluminescence on the nanometer scale in a scanning electron microscope. Spectra were recorded by exciting luminescence along single pillars and across the underlying wetting layer. Luminescence from the excited pillar spots exhibited sharp bound exciton lines and intrinsic free-exciton recombination, whereas the wetting layer showed only very weak luminescence. Along the pillars, virtually no shift of the lines was observed. This is interpreted as evidence for strain-free growth of the pillars. In the bottom region of the pillars aluminum incorporation from the substrate material was found. There are indications for an incorporation of gallium along the pillars due to unintentional doping introduced by the source material. The spot-resolved results are compared to integral photoluminescence measurements with large-area excitation carried out on the same sample region.

Enforced c-axis growth of ZnO epitaxial chemical vapor deposition films on a-plane sapphire

To enforce perfect c-axis orientation of ZnO epitaxial films grown on a-plane sapphire, we first grew perfectly perpendicularly aligned (i.e., c axis oriented) ZnO nanopillars on such sapphire substrates, and then over-grew these by a closed epitaxial film using a modified chemical vapor deposition process at atmospheric pressure. X-ray diffraction and low temperature photoluminescence measurements confirm the desired epitaxial relationship and very high crystalline quality. This growth scheme is an efficient method to suppress dislocations and polycrystalline growth of ZnO and could work equally well for other heteroepitaxial epilayer/substrate systems.

Influence of the measurement procedure on the field-effect dependent conductivity of ZnO nanorods

The electrical properties of field-effect transistors fabricated on the basis of single ZnO nanorods were analyzed under ambient conditions and in the chamber of a scanning electron microscope under high-vacuum conditions. Under ambient conditions, the threshold voltage and conductivity may depend strongly on the details of the measurement procedure as the chosen gate voltage range and gate voltage sweep direction. Electron irradiation in a scanning electron microscope under high-vacuum conditions at ∼10−5 mbar leads to desorption of oxygen and other electronegative molecules, which can increase the conductivity by more than two orders of magnitude.

Growth of zinc oxide nanopillars on an iridium/yttria-stabilized zirconia/silicon substrate

Zinc oxide nanopillars were grown by a self-catalyzed growth process on an epitaxial Ir/yttria-stabilized zirconia/Si(111) multilayer structure in an optically heated tube furnace. The pillars obtained stand upright parallel to each other with their c-axis perpendicular to the sample surface. Problems due to alloying of Zn with Si are completely avoided, and no irregularities of the pillars in the initial growth phase are found. Cathodoluminescence measurements show narrow linewidths below 700μeV due to the excellent crystal quality. Termination of ZnO pillars with a flat metallic iridium layer is an attractive issue towards an optical cavity for laser action.

Incorporation of Ga in ZnO∕GaN epitaxial films

Growth of zinc oxide (ZnO) layers on gallium nitride (GaN) substrates benefits from the small lattice mismatch of these two materials. We report on spatially resolved cathodoluminescence studies of ZnO layers grown by a modified chemical vapor deposition process on GaN templates deposited on sapphire substrates. Line scans across the ZnO∕GaN interface reveal the incorporation of gallium from the template into the ZnO layer. Transmission electron microscopy and micro-Raman measurements both indicate that strain relaxation occurs within a distance of a few nanometers from the ZnO∕GaN interface. The diffusion coefficient of gallium in ZnO is determined.

Structural and cathodoluminescence properties of ZnO nanorods after Ga-implantation and annealing

Single-crystalline ZnO nanorods were implanted with 30 keV Ga+ ions and fluences between 5×1012 and 1.5×1016 cm−2. Annealing treatments at temperatures up to 700 °C for 1 h were carried out to reduce implantation-induced structural defects. The structural and optical properties of the nanorods were studied by transmission electron microscopy (TEM) and cathodoluminescence (CL) spectroscopy. TEM shows that extended implantation defects vanish completely in nanorods implanted with doses up to 5×1013 cm−2 after annealing at 700 °C. Dislocation loops remain after implantation with higher fluences. The CL intensity of as-grown nanorods and implanted ZnO nanorods is low. Annealing at 700 °C leads to a significant increase in the CL intensity for as-grown nanorods and implanted with a dose of up to 5×1013 cm−2. The strong CL intensity in implanted and annealed nanorods correlates with the complete disappearance of extended structural defects.

Epitaxial growth of coaxial GaInN-GaN hetero-nanotubes

We report about the successful realization of a coaxial hetero structure grown by MOVPE around ZnO nanocolumns. At higher overgrowth temperatures, the ZnO cores completely dissolved leaving GaN nanotube structures with excellent properties. Such tubes could be sheathed by a GaInN-GaN single quantum well structure, as confirmed by photoluminescence and transmission electron microscopy.