Polarity in GaN and ZnO: Theory, measurement, growth, and devices

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Rev. 3, 041303 (2016) and may be found at https://doi.org/10.1063/1.4963919.The polar nature of the wurtzite crystalline structure of GaN and ZnO results in the existence of a spontaneous electric polarization within these materials and their associated alloys (Ga,Al,In)N and (Zn,Mg,Cd)O. The polarity has also important consequences on the stability of the different crystallographic surfaces, and this becomes especially important when considering epitaxial growth. Furthermore, the internal polarization fields may adversely affect the properties of optoelectronic devices but is also used as a potential advantage for advanced electronic devices. In this article, polarity-related issues in GaN and ZnO are reviewed, going from theoretical considerations to electronic and optoelectronic devices, through thin film, and nanostructure growth. The necessary theoretical background is first introduced and the stability of the cation and anion polarity surfaces is discussed. For assessing the polarity, one has to make use of specific characterization methods, which are described in detail. Subsequently, the nucleation and growth mechanisms of thin films and nanostructures, including nanowires, are presented, reviewing the specific growth conditions that allow controlling the polarity of such objects. Eventually, the demonstrated and/or expected effects of polarity on the properties and performances of optoelectronic and electronic devices are reported. The present review is intended to yield an in-depth view of some of the hot topics related to polarity in GaN and ZnO, a fast growing subject over the last decade

II-VI Core-Shell Nanowires: Synthesis, Characterizations and Photovoltaic Applications

The emergence of semiconducting nanowires as the new building blocks for photovoltaic (PV) devices has drawn considerable attention because of the great potential of achieving high efficiency and low cost. In special, nanowires with a coaxial structure, namely, core-shell structures have demonstrated significant advantages over other device configurations in terms of radial charge collection and cost reduction. In this dissertation, several core-shell nanowire structures, including ZnO/ZnSe, ZnO/ZnS, and CdSe/ZnTe, have been synthesized and the photovoltaic devices processed from a ZnO/ZnS core-shell nanowire array and a single CdSe/ZnTe core-shell nanowire have been demonstrated. By combining the chemical vapor deposition and pulsed laser deposition (PLD) techniques, type-II heterojunction ZnO/ZnSe and ZnO/ZnS core-shell nanowire array were synthesized on indium-tin-oxide substrates. Their structures and optical properties have been investigated in detail, which revealed that, despite highly mismatched interfaces between the core and shell, both systems exhibited an epitaxial growth relationship. The quenching in photoluminescence but enhancement in photocurrent with faster response upon coating the core with the shell provides the evidence that the charge separation and collection in the type II core-shell nanowire is greatly improved. This demonstration brings much greater flexibility in designing next generation PV devices in terms of material selection and device operation mechanisms for achieving their maximum energy conversion efficiencies at a low cost and in an environmentally friendly manner. In order to achieve a high quality interface in the core-shell nanowire, CdSe and ZnTe, which have close lattice parameters and thermal expansion coefficients, were chosen to fabricate nanowire solar cells. ZnTe and CdSe nanowires were first synthesized by thermal evaporation and the shells were subsequently deposited by PLD. ZnTe/CdSe nanowires represented an inhomogeneous coating while the CdSe/ZnTe core-shell exhibited a conformal coating with obvious ZnTe eptilayer. The final PV device based on an individual CdSe/ZnTe nanowire demonstrated an efficiency of ~1.7%. In addition, a controllable synthesis of CdSe nanowire array on muscovite mica substrate was presented, providing the possibility to harvest hybrid energies in an all-inorganic nanowire array

II-VI Core-Shell Nanowires: Synthesis, Characterizations and Photovoltaic Applications

The emergence of semiconducting nanowires as the new building blocks for photovoltaic (PV) devices has drawn considerable attention because of the great potential of achieving high efficiency and low cost. In special, nanowires with a coaxial structure, namely, core-shell structures have demonstrated significant advantages over other device configurations in terms of radial charge collection and cost reduction. In this dissertation, several core-shell nanowire structures, including ZnO/ZnSe, ZnO/ZnS, and CdSe/ZnTe, have been synthesized and the photovoltaic devices processed from a ZnO/ZnS core-shell nanowire array and a single CdSe/ZnTe core-shell nanowire have been demonstrated. By combining the chemical vapor deposition and pulsed laser deposition (PLD) techniques, type-II heterojunction ZnO/ZnSe and ZnO/ZnS core-shell nanowire array were synthesized on indium-tin-oxide substrates. Their structures and optical properties have been investigated in detail, which revealed that, despite highly mismatched interfaces between the core and shell, both systems exhibited an epitaxial growth relationship. The quenching in photoluminescence but enhancement in photocurrent with faster response upon coating the core with the shell provides the evidence that the charge separation and collection in the type II core-shell nanowire is greatly improved. This demonstration brings much greater flexibility in designing next generation PV devices in terms of material selection and device operation mechanisms for achieving their maximum energy conversion efficiencies at a low cost and in an environmentally friendly manner. In order to achieve a high quality interface in the core-shell nanowire, CdSe and ZnTe, which have close lattice parameters and thermal expansion coefficients, were chosen to fabricate nanowire solar cells. ZnTe and CdSe nanowires were first synthesized by thermal evaporation and the shells were subsequently deposited by PLD. ZnTe/CdSe nanowires represented an inhomogeneous coating while the CdSe/ZnTe core-shell exhibited a conformal coating with obvious ZnTe eptilayer. The final PV device based on an individual CdSe/ZnTe nanowire demonstrated an efficiency of ~1.7%. In addition, a controllable synthesis of CdSe nanowire array on muscovite mica substrate was presented, providing the possibility to harvest hybrid energies in an all-inorganic nanowire array

ZnO Nanostructures: Low-Temperature Synthesis, Characterisation, Their Potential Application as Gene-Delivery Tools

Among metal oxide nanomaterials, zinc oxide (ZnO) nanostructures are one of the most important nanomaterials in today’s nanotechnology research. Over the past several decades, ZnO nanostructures have been extensively investigated for their extraordinary physical and chemical characteristics and also for their prominent performance in various novel applications such as photonics, optics, electronics, drug delivery, cancer treatment, bio-imaging, etc. However, the functionality of theses nanomaterials is eventually dictated by the capability to govern their properties including shape, size, position, and crystalline structure on the nanosized scale. This thesis investigates the solution-based synthesis of ZnO nanostructures and their morphological and structural properties. Importantly, in order to achieve the promising structure of ZnO, a systematic investigation of the influence of processing parameters, including solution concentration, time and temperature of growth reaction on the resultant materials was addressed. The other main point for this work is not only to effectively control the morphology, size, uniformly distribution, and orientation of ZnO nanomaterials, but also to build a good comprehension of the mechanism of the fabrication process to raise their performance in future nanoscale applications. Furthermore, the catalytic effect of RF sputtered gold (Au) thin layer on Si substrate prior to ZnO growth was investigated to demonstrate the contributory for the remarkable catalytic activity of Au nanoparticles in the formation of high-quality ZnO nanostructures. Furthermore, we introduce an effective, inexpensive lithographic patterning method to consistently control the position of solution-processed ZnO nanowires. Nanosphere lithography technique (NSL) utilizes a catalyst-assisted pattern generated by employing colloidal self-assembled crystal of polystyrene spheres (PS) on the substrate surface to guide the hydrothermal growth of ZnO nanowires. Further, we fabricate 3D NFs and branched NFs of ZnO on a silicon substrate via a simple and cost-effective solution growth method, incorporating with seed ZnO nanoparticles deposition. The synthesis of 3D branched ZnO nanostructure could potentially exploit for applications in optoelectronics, catalysis, sensing, and photovoltaics. In addition to the synthesis of 1D and 3D ZnO nanostructures, their morphology and distribution have been analysed via scanning electron microscopy (SEM) while the surface topography was analysed by atomic force microscopy (AFM). The crystalline structure, phase purity, and particle size of ZnO nanomaterials have been investigated using X-ray diffraction (XRD). The outcomes from all these efforts have been integrated for cellular investigation via fluorescence microscopy technique (FM) to demonstrate the potential application of ZnO nanostructures as a gene delivery/-tissue engineering tool in different expression systems.Libyan Governmen

Atomic layer deposition: An enabling technology for the growth of functional nanoscale semiconductors

In this paper, we present the progress in the growth of nanoscale semiconductors grown via atomic layer deposition (ALD). After the adoption by semiconductor chip industry, ALD became a widespread tool to grow functional films and conformal ultra-thin coatings for various applications. Based on self-limiting and ligand-exchange-based surface reactions, ALD enabled the low-temperature growth of nanoscale dielectric, metal, and semiconductor materials. Being able to deposit wafer-scale uniform semiconductor films at relatively low-temperatures, with sub-monolayer thickness control and ultimate conformality, makes ALD attractive for semiconductor device applications. Towards this end, precursors and low-temperature growth recipes are developed to deposit crystalline thin films for compound and elemental semiconductors. Conventional thermal ALD as well as plasma-assisted and radical-enhanced techniques have been exploited to achieve device-compatible film quality. Metal-oxides, III-nitrides, sulfides, and selenides are among the most popular semiconductor material families studied via ALD technology. Besides thin films, ALD can grow nanostructured semiconductors as well using either template-assisted growth methods or bottom-up controlled nucleation mechanisms. Among the demonstrated semiconductor nanostructures are nanoparticles, nano/quantum-dots, nanowires, nanotubes, nanofibers, nanopillars, hollow and core-shell versions of the afore-mentioned nanostructures, and 2D materials including transition metal dichalcogenides and graphene. ALD-grown nanoscale semiconductor materials find applications in a vast amount of applications including functional coatings, catalysis and photocatalysis, renewable energy conversion and storage, chemical sensing, opto-electronics, and flexible electronics. In this review, we give an overview of the current state-of-the-art in ALD-based nanoscale semiconductor research including the already demonstrated and future applications. © 2017 IOP Publishing Ltd

Atomic Layer Deposition Seeded ZnO Nanowires in Hybrid Carbon Fiber Composites: Synthesis, Characterization and Multifunctionality

Interfacial treatments of carbon fiber composites play a critical role in determining the overall performance as the surface of carbon fiber is smooth and inert causing low bonding to the polymer matrix. In this dissertation, atomic layer deposition (ALD) seeded ZnO nanowires were grown on carbon fiber as an enhanced interphase using two-step hydrothermal method for the first time. The effects of growth parameters of seed layers by ALD and nanowire growth in hydrothermal method were systematically investigated. Several morphologies of ZnO nanostructures were obtained and characterized using field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). Single carbon fiber composites and carbon fiber composite laminates with ZnO nanowires were manufactured, then tested by single fiber fragmentation test, 3-point bending test, short beam 3-point bending test. It was found that the incorporation of ZnO nanowires significantly improved the mechanical properties of composites including interfacial shear strength, flexural strength and interlaminar shear strength by up to 286%,45.6% and 31.1%.The successful development and characterization of ZnO nanowires enhanced structural composites have great potential to lead to new generation of lightweight materials with increased mechanical properties for broad mechanical and aerospace engineering applications

Feasibility Study Of Zno Nanorods For Sensor Application

A field study was conducted to evaluate the effects of three insecticides (abamectin, malathion and diafenthiuron) on the interactions between Bemisia tabaci (Homoptera: Aleyrodidae) the brinjal pest, and the parasitoid Encarsia hitam (Hymenoptera: Aphelinidae), as well as on plant performances and fruit production. All insecticides were applied weekly at recommended doses over two brinjal cropping periods. Overall results showed high total numbers of whitefly on untreated plants in the first (14.08 ± 1.44 per leaf) and second (17.50 ± 4.65 per leaf) cropping periods compared to plants that were treated with abamectin (13.88 ± 3.32), malathion (9.80 ± 2.19 per leaf) and diafenthiuron (10.40 ± 2.41 per leaf) in the first crop and in second crop with 15.65 ± 5.42, 8.35 ± 2.79 and 9.48 ± 2.3 per leaf respectively. Malathion and diafenthiuron were the most effective insecticides that reduced whitefly populations below economic threshold level (ETL). Percentages of parasitism was high on whitefly nymphs in untreated control plants in the first and second cropping periods (3.17% - 12.82%) and (0.52% - 10.00%) respectively compared with insecticide-treated plants (0.62% - 12.50%) and (1.01% - 8.00%) respectively. All insecticides affected the parasitization of whitefly although no significance difference was observed among treatments