M. Al-Suleiman

Zinc oxide nanorod based photonic devices: recent progress in growth, light?emitting diodes and lasers

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.

Photoluminescence properties: Catalyst-free ZnO nanorods and layers versus bulk ZnO

In this contribution, we compare the photoluminescence properties of ZnO nanorods and epilayers with those of bulk ZnO. Owing to the high aspect ratio (length of 4–14μm, diameter of 80–500nm), the characterized ZnO nanorods show very good optical properties. Due to the high surface-to-volume ratio in ZnO nanorods, surface excitons dominate at low temperature. The optical properties of nanorod ensembles improve with increasing nanorod length. The photoluminescence emission from free A excitons was intense in the ZnO layer at 13K.

Optical investigations and exciton localization in high quality Zn1−xMgxO–ZnO single quantum wells

This work investigates the photoluminescence properties of Zn1−xMgxO–ZnO single quantum wells, which have been fabricated by molecular-beam epitaxy. With increasing temperature from 13to300K the single quantum well-related emission peaks exhibit an irregular S-shaped (redshift-blueshift-redshift) behavior, which is in contrast with that ascribed to band gap shrinkage (redshift). In order to clarify the origin of this behavior, the temperature dependence of the integral photoluminescence intensity of the quantum well emission was studied and the relevant activation energies were calculated and correlated to its full width at half maximum, band offsets, and monolayer fluctuations.

Recombination dynamics and lasing in ZnO/ZnMgO single quantum well structures

We report a time-resolved study of the recombination dynamics in molecular beam epitaxy grown ZnO∕ZnMgO single quantum wells (SQWs) of 1.0–4.5nm width. The SQWs exhibit different emission properties, depending on both the well width and defect density. Stimulated emission has been achieved at room temperature in a separate confinement double heterostructure having a 3nm wide SQW as an active region. It has been found that a critical parameter for the lasing is the inhomogeneous broadening of both QW and barrier emission bands.

Mechanisms for high internal quantum efficiency of ZnO nanorods

We investigate the photoluminescence properties of ZnO nanorods grown by vapor phase epitaxy. The room temperature photoluminescence spectra exhibit an energy shift of about 80 meV to lower energy in comparison with that of bulk ZnO as well as ZnO epilayers. The emission band observed at 3.31 eV at low temperature dominates the photoluminescence at room temperature. The high internal quantum efficiency of about 33% is explained in terms of this band, which seems to result from excitons bound to surface defect states.

Properties of V-implanted ZnO nanorods

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.

MOVPE gallium-nitride nanostructures fabricated on ZnO nanorod templates grown from aqueous chemical solution

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.