ABSTRACT
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.
ABSTRACT
Low-area density ZnO nanowire arrays, growing perpendicularly to the substrate, are synthesized with high-pressure pulsed laser deposition. The introduction of a ZnO buffer layer enables us to fabricate individual nanowires several micrometres apart (area density<0.1 nanowire microm(-2)), suppressing any shadowing effect by neighbouring nanowires during subsequent growth. These low density ZnO nanowires, whose c-axis is perpendicular to the substrate surface, are then used as templates to grow ZnO/ZnMgO core-shell nanowire heterostructures with conventional low-pressure pulsed laser deposition. Cathodoluminescence spectroscopy as well as transmission electron microscopy show that a sharp interface forms between the ZnO core and the ZnMgO shell. Based on these findings, we have grown a series of radial ZnO/ZnMgO quantum wells with different thicknesses that exhibit quantum confinement effects, with thicker quantum wells emitting at lower energies. Spatially resolved cathodoluminescence confirms the homogeneity of the quantum well structure along the full nanowire length of about 3 microm.
ABSTRACT
MgZnO/ZnO quantum wells on top of ZnO nanowires were grown by pulsed laser deposition. Ensembles of spatially fluctuating and narrow cathodoluminescence peaks with single widths down to 1 meV were found at the spectral position of the quantum well emission at 4 K. In addition, the number of these narrow QW peaks increases with increasing excitation power in micro-photoluminescence, thus pointing to quantum-dot-like emission centers. Indeed, laterally strained areas of about 5 nm diameter were identified at the quantum well positions on top of the nanowires by high-resolution transmission electron microscopy.