Concerning optoelectronic devices fabricated by epitaxial methods, the combination of ZnO and GaN has promising aspects regarding their good optical properties and a relatively good lattice matching between both as compared to other foreign substrates like sapphire or silicon. Moreover ZnO nanopillar arrays may serve as a template for GaN nanopillar fabrication or for high quality GaN layers by lateral overgrowth of the ZnO nanopillars. In this work, we investigate the combination of two very different growth methods – aqueous chemical low temperature growth (ACG) for the ZnO nanopillar templates on silicon substrates and metalorganic vapor phase epitaxy (MOVPE) for the GaN overgrowth – in order to show to which extent the very cost efficient ZnO templates suit the high demands of GaN MOVPE. By a combination of annealing and photoluminescence experiments we show that the properties of the heterostructures change significantly with temperature.

Zinc oxide (ZnO), with its excellent luminescent properties and the ease of growth of its nanostructures, holds promise for the development of photonic devices. The recent advances in growth of ZnO nanorods are discussed. Results from both low temperature and high temperature growth approaches are presented. The techniques which are presented include metal-organic chemical vapour deposition (MOCVD), vapour phase epitaxy (VPE), pulse laser deposition (PLD), vapour-liquid-solid (VLS), aqueous chemical growth (ACG) and finally the electrodeposition technique as an example of a selective growth approach. Results from structural as well as optical properties of a variety of ZnO nanorods are shown and analysed using different techniques, including high resolution transmission electron microscopy (HR-TEM), scanning electron microscopy (SEM), photoluminescence (PL) and cathodoluminescence (CL), for both room temperature and for low temperature performance. These results indicate that the grown ZnO nanorods possess reproducible and interesting optical properties. Results on obtaining p-type doping in ZnO micro- and nanorods are also demonstrated using PLD. Three independent indications were found for p-type conducting, phosphorus-doped ZnO nanorods: first, acceptor-related CL peaks, second, opposite transfer characteristics of back-gate field effect transistors using undoped and phosphorus doped wire channels, and finally, rectifying I-V characteristics of ZnO: P nanowire/ZnO:Ga p-n junctions. Then light emitting diodes (LEDs) based on n-ZnO nanorods combined with different technologies (hybrid technologies) are suggested and the recent electrical, as well as electro-optical, characteristics of these LEDs are shown and discussed. The hybrid LEDs reviewed and discussed here are mainly presented for two groups: those based on n-ZnO nanorods and p-type crystalline substrates, and those based on n-ZnO nanorods and p-type amorphous substrates. Promising electroluminescence characteristics aimed at the development of white LEDs are demonstrated. Although some of the presented LEDs show visible emission for applied biases in excess of 10 V, optimized structures are expected to provide the same emission at much lower voltage. Finally, lasing from ZnO nanorods is briefly reviewed. An example of a recent whispering gallery mode (WGM) lasing from ZnO is demonstrated as a way to enhance the stimulated emission from small size structures.

We report the electrochemical growth of zinc oxide nanorods in a zinc nitrate/hexamethylenetetramine solution at 70 °C. High-density vertical nanorods were grown on Au films on silicon substrates with a texture coefficient better than 99.9%. By varying the reactant concentration the diameter can be varied between 100 and 250 nm, with corresponding lengths of 1 to 4 μm. Furthermore, this approach was used for the selective growth on Ti/Au strip conductors ordered in an interdigitated structure on an insulating substrate. We achieved the growth of ZnO nanorods between neighbouring strip conductors bridging the gap between them. In this configuration the nanorods are already contacted and electrical measurements can be directly performed. First I–V measurements show a good conductivity of the as-grown nanorods and the resistance could be estimated to be 0.1 Ω cm. Under UV illumination the ZnO nanorods demonstrate a photoconductivity, but only after annealing the sample at 300 °C in N2.

We have deposited and characterised ZnO on flexible and transparent plastic polymer. We employed a specially designed vapour phase growth system with elemental sources for zinc and oxygen and deposited thin ZnO films at temperatures below 400 degrees C. Basic photoluminescence characterisation confirms ZnO. Ohmic contacts were fabricated on these layers and the layers exhibit significantly high electron concentration with carrier mobility mu of up to 10.78 cm(2) V-1 s(-1). Furthermore, we show how these layers can be processed with conventional device processing techniques. (c) 2007 Elsevier B.V. All rights reserved.

The optical properties of ZnO nanorods realized by an advanced low-temperature aqueous chemical growth on both silicon and plastic substrates are presented. Systematic photoluminescence investigations in the temperature range of 4–293K reveal strong and well-resolved near-band-edge emission even for rods on plastic substrate, and a weak deep-level emission. At intermediate temperatures phonon replicas of excitonic lines are observable. The optimum molar concentration range of the solution for obtaining nanorods of good optical quality is shown to lie between 0.025M and 0.075M. The large linewidth of the near-band-edge emission (∼10meV), its temperature dependence, and the absence of sharp excitonic transitions indicate that this emission is a result of transitions from a band of donor states.

An overview of our nanostructures and thin layers fabrication technologies including MOCVD, vapour phase transport, MBE, chemical growth, electrodeposition is presented. The developed technologies provide control on the nanopillar characteristics, and still leaving the self-organisation mechanism untouched. Also fabrication of complicated heterostructures like quantum wells both as epitaxial layers or embedded into nanopillars was demonstrated. First results of the device applications of ZnO nanostructures as well as implementation of alternative ZnO nanostructures like nanopowders are discussed.

We report on the fabrication of solid-state dye-sensitized solar cells with ZnO nanorods as n-type material and CuSCN as p-type material. The ZnO nanorods were grown on ITO-coated glass by using a low-temperature aqueous chemical growth approach employing different growth times. If the growth time is changed, the nanorod morphology can be varied in lentgh and diameter. CuSCN was deposited on the dye-coated ZnO nanorods from a solution in di-n-propyl sulphide, so that the nanorods are fully embedded in CuSCN. The ZnO/dye/CuSCN solar cells show photocurrents of 0.26 mA/cm(2), an open circuit voltage of 0.34 V and an energy conversion efficiency of 0.1 %.

Photoluminescence studies have been carried out on ZnO nanorods from an aqueous chemical growth. The results reveal well‐resolved near‐band‐edge emission with a broad linewidth of about 10 meV. Nanorods of different lengths and diameters on Si and even on plastic substrates showed similar optical properties indicating a good control of the growth process without much influence on the optical properties. Time‐resolved photoluminescence measurements show a very fast decay time of about 70 ps for the as‐grown samples but longer decay time of about 175 ps for the annealed samples.

ZnO nanorod arrays find applications in solar energy conversion, light emission and other promising areas. One approach to generate ZnO nanorods is the cost efficient aqueous chemical growth (ACG). Usually the ACG process is based on a nucleation step followed by growth of ZnO nanorods in aqueous solution at temperatures below 95 degrees C.We report on the fabrication of homogeneous, large scale arrays of nanorods on various substrate materials (Si, glass, polymer) by ACG. PL-measurements show surprisingly good optical quality although the rods were grown at low temperature.Even though we have developed patterning of these arrays with photolithographic techniques, a bottom up approach for lateral patterning is important concerning further applications especially for mass-production. The substrates with patterned metal layers were employed to realize selective growth of nanorods. The experiments were carried out on Ti-, Ag- and Pt-patterned substrates. Selective growth on metal structured glass substrates was developed and is described. (c) 2007 Elsevier Ltd. All rights reserved.

The influence of ZnO seed crystals and postgrowth annealing on low-temperature aqueous chemically grown ZnO nanorods is analyzed. At the seed crystal/nanorod interface a high density of structural defects leads to emission at 3.332 eV, attributed to excitons bound to structural defects. This peak is absent for seed crystals, very pronounced for rods of shorter lengths grown on seed crystals, and reduced for longer nanorods. After annealing in oxygen and nitrogen atmosphere, the near-band-edge excitonic transitions sharpen and deep-level emission is strongly reduced. Time-resolved photoluminescence measurements show a striking similarity between donor-bound excitons and excitons bound to structural defects.

Aqueous chemical growth (ACG) is an efficient way to generate wafer-scale and densely packed arrays of ZnO nanopillars on various substrate materials. ACG is a low-temperature growth approach that is only weakly influenced by the substrate and even allows growth on flexible polymer substrates or on conducting materials. The advanced fabrication of wafer-scale and highly vertically aligned arrays of ZnO nanopillars on various substrate materials is demonstrated. Moreover, it is possible to control the morphology in diameter and length by changing the growth conditions. Photoluminescence characterization clearly shows a comparatively strong band-edge luminescence, even at room temperature, that is accompanied by a rather weak visible luminescence in the yellow/orange spectral range.

ZnO nanorods were grown on Si substrates by an aqueous chemical approach and subsequently doped by V implantation. Transmission electron microscopy and photoluminescence spectroscopy reveal a severely defective material directly after the implantation process. Subsequent annealing leads to a partial recovery of the crystal structure. The magnetic features of ZnO:V nanorods were investigated by magnetic force microscopy. Images taken of ensembles as well as of single rods clearly display contrast, which is seen as a strong indication of ferromagnetism at room temperature.

The magnetic properties of vanadium doped ZnO nanorods formed by different growth methods were examined. The samples under investigation were either grown by metallorganic vapour-phase epitaxy (MOVPE) with in situ incorporation of the V atoms or by aqueous chemical growth (ACG) in combination with an ion-implantation process. The V concentration for all samples was less than 2 at.%.Field-cooled SQUID measurements only reveal a weak ferromagnetic signal still in a very sensitive measuring range. However, the MFM micrographs obtained at room temperature show a clear magnetic contrast and complex structures, the images are in good agreement with the phase images expected for vertically aligned magnetic dipoles. This is a strong indication for a ferromagnetic behaviour at room temperature. (c) 2007 Elsevier Ltd. All rights reserved.

Zinc oxide (ZnO) nanorods were grown by a wet chemical approach and by vapor phase transport. To explore the electrical properties of individual nanostructures current–voltage (I –V) characteristics were obtained by using an atomic force microscope (AFM) with a conductive tip or by detaching the nanorods from the growth substrate, transferring them to an isolating substrate and contacting them with evaporated Ti/Au electrodes patterned by electron‐beam lithography. The AFM‐approach only yields a Schottky diode behavior, while the Ti/Au forms ohmic contacts to the ZnO. For the latter method the obtained I –V curves reveal a resistivity of the nanorods in the order of 10–5 Ω cm which is unusually low for undoped ZnO. We therefore assume the existence of a highly conductive surface channel. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence investigations of ZnO nanorods realised by an advanced two-step aqueous chemical growth process have been carried out revealing well-resolved near-band-edge emission accompanied by phonon replicas. The optical properties of nanorods with different lengths and diameters are quite similar indicating a good control of the growth process without influencing the optical properties even on plastic substrate. The near-band-edge emission has a very broad line-width of similar to 10 meV. Annealing in Ar atmosphere reduces the deep-level emission with a corresponding increase of the near-band-edge emission. (c) 2007 Elsevier Ltd. All rights reserved.

Aqueous chemical growth (ACG) is a low-temperature approach that is only weakly influenced by the substrate and allows for the growth of ZnO nanopillars on various substrates. ACG is an efficient way to generate wafer-scale and densely packed arrays of ZnO nanopillars even on polymer materials. Photoluminescence (PL) characterisation clearly shows a comparatively strong band-edge luminescence even at room temperature that is accompanied with a rather weak visible luminescence in the yellow / orange spectral range. We introduce a rather simple postgrowth lithographic technique. Patterning of ZnO nanopillars even on layered conducting and flexible substrate materials using ACG as a low-temperature growth technique is demonstrated. The economical potential for future applications and devices using ZnO nanopillar arrays is discussed. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.