A. El-Shaer

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.

Magnetic property investigations on Mn-doped ZnO Layers on sapphire

The need for diluted magnetic semiconductors has stimulated research on Mn-doped ZnO. However, the type of magnetic coupling (ferro/para) in ZnMnO remains an issue of debate. We have investigated the magnetic properties of Mn-doped ZnO layers grown by molecular beam epitaxy. Some samples showed a hysteresis with remnant magnetization on the order of 10−5emu, thus eventually suggesting ferromagnetism. We observed that the critical influence of the substrate substantially affects magnetic property measurements. This has to be taken into account in order to clearly confirm ferromagnetism. In our case, after subtraction of the substrate effect, there is no evidence of a ferromagnetic behavior for the ZnMnO samples.

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.

Structural characterization of ZnO films grown by molecular beam epitaxy on sapphire with MgO buffer

The structural characteristics of the ZnO film grown on sapphire substrate using a thin MgO buffer layer were studied using transmission electron microscopy and high-resolution x-ray diffraction. The growth was carried out in a modified plasma-molecular beam epitaxy system. The observed misfit dislocations were well confined at the sapphire overgrown interface exhibiting domain matching epitaxy, where the integral multiples of lattice constants match across the interface. The main extended defects in the ZnO film were the threading dislocations having a mean density of 4×109cm−2. The formation of the MgO buffer layer as well as the ZnO growth were monitored in situ by reflection high-energy electron diffraction. The very thin ∼1nm, MgO buffer layer can partially interdiffuse with the ZnO as well as react with the Al2O3 substrate forming an intermediate epitaxial layer having the spinel (MgO∕Al2O3) structure.

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.

Demonstration of an ultraviolet ZnO-based optically pumped third order distributed feedback laser

The authors demonstrate an optically pumped ZnO distributed feedback laser operating at 383nm. For a large temperature range between 10 and 270K, the device lased in a single longitudinal mode. Mode selection was accomplished via a third order diffraction grating, which was dry etched into a 120nm thick Si3N4 layer deposited on the ZnO active region. They observed a spectral linewidth of 0.4nm, a pump threshold intensity of 0.12MW∕cm2, and a peak output power of 14mW. From wavelength versus temperature measurements, they deduced a temperature tuning coefficient of the ZnO refractive index of 9×10−5K−1.

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.

Optical Applications of ZnO Nanowires

This paper discusses different aspects of optical applications of ZnO nanowires NWs. After a description of the relevant synthesis and fabrication techniques, light-emitting diodes based on ZnO NW and NW arrays are introduced and different experimental realizations from the literature are discussed. The working principle of ZnO UV photodetectors is presented, and improvements and limitations of ZnO- NW-based dye-sensitized solar cells are discussed. Different aspects of ZnO-NW waveguides and their potential application for biological sensing are described. Finally, the current status of ZnO-NW-based UV lasers is presented.

Etch-Pit Density Investigation on Both Polar Faces of ZnO Substrates

It is well known that the presence of dislocations in electronic and optoelectronic devices based on semiconductor p-n junctions can lead to short circuits of these junctions and thus to malfunction. The main cause of dislocations in homoepitaxial layers is threading dislocations present in the initial substrate. Thus, it is extremely important to investigate defect structure, particularly dislocations appearing on the surface of commercial wafers, which are employed for epitaxial growth. Zinc oxide is a very promising material for applications in thermally stable optoelectronic devices, such as light-emitting diodes and laser diodes emitting in the ultraviolet spectral region, mostly because of its high exciton-binding energy 60 meV. Much work has been done on the heteroepitaxy of ZnO, for example, on sapphire, in order to get good-quality epitaxial layers. However, due to problems associated with heteroepitaxy of ZnO, such as high dislocations density as a result of lattice mismatch and diffusion of Al atoms into epitaxial layers in the case of growth on sapphire, obtaining p-type ZnO has rather proven to be challenging. Great attention is now being focused on ZnO substrates with the intention to realize homoepitaxially grown p-doped layers. This technology has not yet been substantially developed and it is still not clear on which polar face of ZnO substrates p-doping can be efficiently and reproducibly realized. In this situation, both zinc- and oxygen-polar faces of substrates have to be characterized. In the case of compound semiconductors with polar faces, revealing etch pits by chemical etching on both faces is typically not possible. 1 It was shown that zinc- and oxygen-terminated sides of ZnO bulk wafers are influenced by acids in drastically different ways. 2 This behavior was explained in terms of the surface-bonding model. The etching rate of oxygen-terminated ZnO is much higher than that of zinc-terminated ZnO. The morphologies are also qualitatively different after chemical treatment. The etching of oxygenterminated ZnO results in the formation of so-called hillocks, which do not provide any useful information about the structure of defects. In contrast to these hillocks, etch pits appear on the opposite polar side. Revealing these etch pits by wet-chemical etching is normally used to define the density of threading dislocations on zinc face. As an alternative to wet-chemical etching, it is possible to employ thermal treatment. Gu et al. showed the appearance of atomic terraces after annealing in air for 3 h at 1050°C. 3 Graubner et al. state that annealing in an oxygen environment causes the formation of atomic terraces only at temperatures higher than 1100°C. 4 In 1971, the appearance of hexagonally formed etch pits on an oxygen face of ZnO bulk wafer after thermal treatment in helium atmosphere at 1200°C was reported by Wolf et al. 5 After considering these reports, questions about the dynamics of the thermal treatment process arise. The influence of the gas environment and temperature effect are not clear. In this work, we present the results on chemical and thermal etching of ZnO substrates supplied by two companies: TokyoDenpa and Crystec. The first approach was used to reveal etch pits on a zinc-polar face and the latter one on an oxygen-polar face. The thermal treatment under special conditions was also used to improve the morphology of ZnO substrates. Experimental For chemical etching, the following acids and their solutions were used: 10% and 1% solutions of HCl, 15% and 1% solutions of H3PO4, 20% and 1% solutions of HNO3, and a 1% solution of CH3COOH. In most cases, the samples from both suppliers were etched at room temperature. The etching process with acetic acid was conducted at room temperature and at 5°C. The etching durations were specially optimized in each case. The samples were treated in 10% and 1% HCl for 6 s, in a 15% solution of H3PO4 for 40 s, and in 1% H3PO4 for 5 s. The etching duration in 20% HNO3 was 90 s and in 1% HNO3, 30 s. The smallest etching duration 2s was for CH3COOH. Thermal treatment was done at 1050, 1100, and 1150°C, for durations from 1 to 5 h. The samples from Crystec were annealed at 1050°C in quartz‐glass oven in excess air and nitrogen for 3 and 4 h. Crystec substrates were also thermally etched at 1100°C for 3, 4, and 5 h and at 1150°C for 1, 2, and 3 h. Atomic force microscopy AFM measurements were done with a DME microscope in alternating current ac mode before and after chemical and thermal treatment.

Demonstration of a ZnO/MgZnO-based one-dimensional photonic crystal multiquantum well laser

A ZnO/MgZnO-based one-dimensional photonic crystal multiquantum well laser operating at an emission wavelength of 360.7 nm is demonstrated. The photonic crystal providing optical feedback was fabricated in the form of parallel grooves with a period of 277.3 nm and a depth of 100 nm in a Si3N4 layer deposited directly on the epitaxial material. At a temperature of 11 K, 16 mW peak power is emitted from the laser surface, and the threshold intensity amounts to 0.33 MW/cm2. From temperature-dependent output power versus pump intensity curves, we deduced a T0 of 60 K and a maximal operating temperature of 135 K.