Author: Vladimir Petukhov
Publisher:
ISBN: 9783954040841
Category :
Languages : en
Pages : 101

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Author: Abdelhamid Abdelrehim Mahmoud Elshaer
Publisher: Cuvillier Verlag
ISBN: 3736927010
Category : Computers
Languages : en
Pages : 144

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In semiconductor research a reliable epitaxial growth technique for growing high quality thin films and heterostructures is necessary. In the case of ZnO one of the main difficulties is the absence of suitable substrate material for ZnO epitaxial growth. Although special oxide material (for example ScAlMgO4) and ZnO bulk crystal can serve as lattice matched substrates, the quality of the substrates themselves, the size of the available wafer, and the expense do not encourage to use these lattice matched substrates for ZnO epitaxial growth. In the current research, a widely used low cost commercial substrate sapphire was employed to develop a reliable epitaxial growth technique and growth process for ZnO. The versatile epitaxial growth technique, molecular beam epitaxy (MBE) equipped with a rf-plasma source was developed for growth and various characterizations methods were conducted to obtain a fundamental understanding in both the epitaxial processes and material properties of ZnO thin films and heterostructures. Employing a thin HT MgO buffer layer prior to ZnO growth is the key to overcome the very large mismatches between c-Al2O3 substrate. Wetting the surface of Al2O3 substrate with a few MgO monolayers, lowed the surface energy, so that the lateral growth of ZnO is promoted at the initial growth stage. MgO can be grown in the same chamber as ZnO without any contamination problem. These advantages make the growth procedure of a HT MgO buffer fast and easy. The growth temperature and the growth rate of MgO buffer are found to be important to improve the ZnO heteroepitaxy. An intermediate spinel layer in epitaxial relation with the sapphire substrate as well as with the HT MgO buffer layer is formed in the early stage of growth during the deposition of the MgO at 700°C. It was found that the combination of these two layers is useful for the progressive reduction of the ZnO overgrown with the sapphire substrate.Annealing experiments reveal that as soon as the spinel layer is formed at about 700°C, it remains stable at least up to 1000°C, and even it is extended in thickness. By recording and analyzing RHEED intensity oscillations, the growth kinetics has been investigated. Flat surface morphology and layer-by-layer growth has been achieved. The stoichiometry has been deduced by analyzing the growth rate as a function of Zn and O fluxes for various growth temperatures. It is found that the sticking coefficient of oxygen radicals is less dependent on the substrate temperature than that of Zn. The stoichiometric condition shifts to larger Zn flux at higher growth temperature. The kink rZnO values determine the activated O-flux supplied by the RF plasma source at TS=500°C, 400W and a given O2-flow rate. It equals 0.5±0.05 Ås-1 per sccm. Absolute αZn values versus TS, defined as αZn=rZnO(T)/rZnO(max), where rZnO(max) is recalculated from the Zn flux measured by a quartz monitor, using Zn/ZnO molar mass and density ratios. Ex-situ characterization of the grown ZnO layers indicate that the surface morphology and crystal quality of the ZnO films grown on sapphire by MBE using either oxygen plasma cell or H2O2 as an oxidant can be extensively improved by using an HT MgO buffer. ZnO layers reveal strong variation of surface morphology versus the O/Zn flux ratio. The most flat surface morphology of ZnO is obtained when the ratio is within the 0.7-1 range. The growth under O-rich conditions leads to formation of hexagonal pyramids and at higher O/Zn ratios to a 3D growth with the top layer formed by perfectly c-oriented columnar structures of 50-100 nm in a diameter. It was also possible to recover the initial 3D growth mode to the 2D one by employing the Zn-rich growth conditions at O/Zn=0.4-0.6. Structural characterizations by high resolution X-ray diffraction (HR-XRD) and transmission electron microscopy (TEM) indicate a dramatic reduction in defect density in the ZnO epilayers grown with an HT MgO buffer. By using TEM, it was found that the dominant extending defects are edge, screw and mixed-type dislocations along c-axis. The main defects were threading dislocations. This is resulted from the well controlled layer-by-layer growth, since only the edge-type dislocation is able to accommodate the lattice mismatch, while the screw type dislocation forms much related to the initial nucleation environment.The microstructure of ZnO epilayers has been studied by HR-XRD. The full width at half maximum of the (0002) reflection, 0.007 degree, is much smaller than that of the (10-10) reflection, 0.27 degree revealing the micro-twist dominates the mosaicity, while micro-tilt is much less important.This pronounced difference of the rocking curve widths between the (0002) and (1010) reflections strongly indicates that the density of pure edge threading dislocations is greater than that of pure screw dislocations. Optical characterizations reveal that exciton plays an important role in ZnO. At room temperature free exciton recombinations dominate the photoluminescence. The ZnO epilayers reveal well resolved low temperature PL excitonic spectra with a dominant bound exciton line (3.355 eV) possessing a ~2 meV half-width and a peak of free A exciton at 3.374 eV. The low-energy tail extending from the excitonic emission peaks due to the lattice deformation is significantly reduced, which allows the observation of two electron satellites and LO-phonons replicas of free and bound excitons. Variation of growth stoichiometry from O-rich to Zn-rich results in the pronounced quench of the acceptor-bound part of the excitonic band, as well as the strong intensity redistribution of donor-bound lines which seems to be attributed to a change in the point defect density. Temperature dependence of PL spectra between 6K and room temperature every 30 K under the same excitation conditions was performed. Slowly decreases coming at 300K to about one third of the intensity at 6K. This corresponds to the activation of non-radiative channels in the capturing and recombination processes. This result was confirm by decay time measurements. PL mapping of 2 inch ZnO epilayer shows high lateral homogeneity from PL intensity distribution and PL FWHM distribution. Hall-effect measurements and Electrochemical profiling (ECV) were used to characterize the electrical properties of ZnO samples. Hall-effect measurements indicated n-type behavior with carrier concentration of 2.0x1016 cm-3 and mobility of approximately 96 cm2/Vs. ECV profile versus depth measured for the top 2.5 μm thick sample gives surface carrier concentration is 2.0x1016 cm-3 increasing to a maximum value of 1.0x1018 cm-3 the semiconductor/substrate interface. P-n heterojuntions and mesa structures comprising MBE n-ZnO layers and CVD p-4H-SiC laser were manufactured and investigated. Electrical properties of the mesa diodes have been studied with Hall measurements, and current-voltage measurements (I-V). I-V measurements of the device show good rectifying behavior, from which a turn-on voltage of about 2 V was obtained. With the excitation of O and N gas mixture in a single plasma cell, followed by the sample annealing procedure. P-type ZnO:N layers with a net hole concentration 3x1017 cm-3 using was measured. The combination of low growth temperature, slightly O-rich conditions and post-growth annealing is shown to be effective way to obtain p-doping. Further efforts are necessary to improve structural quality of the low-temperature p-type ZnO:N films. Optical properties of ZnO based II-VI heterostructures and quantum structures have also been studied. The surface roughness of ZnxMg1−xO was as low as 0.7 nm. The optical band gap and photoluminescence peak can be turned to larger energy with the same high crystallinity and without significant change in the lattice constant. The prominent PL peaks related to the SQW show a systematic blueshift with decreasing well width, which is consistent with the quantum size effect. The SQW-related emission peaks exhibit an S-shaped (redshift-blueshiftredshift) behaviour with increasing temperature, which is in contrast with that ascribed to band gap shrinkage (redshift). The observed behavior is discussed in terms of localization at lateral interface potential fluctuations. For T >70 K the integrated PL intensity is thermally activated with activation energies much less than the band offsets. It is argued that the dominant mechanism leading to the quenching of the ZnO SQW-related PL is due to the thermionic emission of excitons out of the lateral potential minima caused by potential fluctuations, such as interface fluctuations by 1 ML. Stimulated emission has been achieved at room temperature in a separate confinement double heterostructure having a 3 nm 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. MBE process for ZnO has been developed where high quality ZnO epilayers and heterostructures can be grown by molecular beam epitaxy on sapphire substrate. For nitrogen doping of ZnO, Oxygen and nitrogen were activated in the single plasma cell. No reproducible and reliable experimental results on the achievement of p-type conductivity achieved. Stimulated emission has been achieved at room temperature.

Author: Zheng Yang
Publisher:
ISBN:
Category : Photoluminescence
Languages : en
Pages : 149

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Doping in zinc oxide (ZnO) thin films is discussed in this dissertation. The optimizations of undoped ZnO thin film growth using molecular-beam epitaxy (MBE) are discussed. The effect of the oxygen ECR plasma power on the growth rate, structural, electrical, and optical properties of the ZnO thin films were studied. It was found that larger ECR power leads to higher growth rate, better crystallinity, lower electron carrier concentration, larger resistivity, and smaller density of non-radiative luminescence centers in the ZnO thin films. Low-temperature photoluminescence (PL) measurements were carried out in undoped and Ga doped ZnO thin films grown by molecular-beam epitaxy. As the carrier concentration increases from 1.8 x 10 to 1.8 x 10 cm -3, the dominant PL line at 9 K changes from I 1 (3.368 - 3.371 eV), to I DA (3.317 - 3.321 eV), and finally to I 8 (3.359 eV). The dominance of I, due to ionized donor bound excitons, is unexpected in n-type samples, but is shown to be consistent with the temperature-dependent Hall fitting results. We also show that I DA has characteristics of a donor acceptor pair transition, and use a detailed, quantitative analysis to argue that it arises from Ga Zn donors paired with Zn-vacancy (V Zn) acceptors. In this analysis, the Ga Zn 0/+ energy is well-known from two-electron satellite transitions, and the V Zn 0/- energy is taken from a recent theoretical calculation. Typical behaviors of Sb-doped p -type ZnO are presented. The Sb doping mechanisms and preference in ZnO are discussed. Diluted magnetic semiconducting ZnO:Co thin films with above room-temperature T C were prepared. Transmission electron microscopy and x-ray diffraction studies indicate the ZnO:Co thin films are free of secondary phases. The magnetization of the ZnO:Co thin films shows a free electron carrier concentration dependence, which increases dramatically when the free electron carrier concentration exceeds ~10 19 cm -3, indicating a carrier-mediated mechanism for ferromagnetism. The anomalous Hall effect was observed in the ZnO:Co thin films. The anomalous Hall coefficient and its dependence on longitudinal resistivity were analyzed. The presence of a side-jump contribution further supports an intrinsic origin for ferromagnetism in ZnO:Co thin films. These observations together with the magnetic anisotropy and magnetoresistance results, supports an intrinsic carrier-mediated mechanism for ferromagnetic exchange in ZnO:Co diluted magnetic semiconductor materials. Well-above room temperature and electron-concentration dependent ferromagnetism was observed in n -type ZnO:Mn films, indicating long-range ferromagnetic order. Magnetic anisotropy was also observed in these ZnO:Mn films, which is another indication for intrinsic ferromagnetism. The electron-mediated ferromagnetism in n -type ZnO:Mn contradicts the existing theory that the magnetic exchange in ZnO:Mn materials is mediated by holes. Microstructural studies using transmission electron microscopy were performed on a ZnO:Mn diluted magnetic semiconductor thin film. The high-resolution imaging and electron diffraction reveal that the ZnO:Mn thin film has a high structual quality and is free of clustering/segregated phases. High-angle annular dark field imaging and x-ray diffraction patterns further support the absence of phase segregation in the film. Magnetotransport was studied on the ZnO:Mn samples, and from these measurements, the temperature dependence of the resistivity and magnetoresistance, electron carrier concentration, and anomalous Hall coefficient of the sample is discussed. The anomalous Hall coefficient depends on the resistivity, and from this relation, the presence of the quadratic dependence term supports the intrinsic spin-obit origin of the anomalous Hall effect in the ZnO:Mn thin film.

Author: Stuart Irvine
Publisher: John Wiley & Sons
ISBN: 1119313015
Category : Technology & Engineering
Languages : en
Pages : 582

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Systematically discusses the growth method, material properties, and applications for key semiconductor materials MOVPE is a chemical vapor deposition technique that produces single or polycrystalline thin films. As one of the key epitaxial growth technologies, it produces layers that form the basis of many optoelectronic components including mobile phone components (GaAs), semiconductor lasers and LEDs (III-Vs, nitrides), optical communications (oxides), infrared detectors, photovoltaics (II-IV materials), etc. Featuring contributions by an international group of academics and industrialists, this book looks at the fundamentals of MOVPE and the key areas of equipment/safety, precursor chemicals, and growth monitoring. It covers the most important materials from III-V and II-VI compounds to quantum dots and nanowires, including sulfides and selenides and oxides/ceramics. Sections in every chapter of Metalorganic Vapor Phase Epitaxy (MOVPE): Growth, Materials Properties and Applications cover the growth of the particular materials system, the properties of the resultant material, and its applications. The book offers information on arsenides, phosphides, and antimonides; nitrides; lattice-mismatched growth; CdTe, MCT (mercury cadmium telluride); ZnO and related materials; equipment and safety; and more. It also offers a chapter that looks at the future of the technique. Covers, in order, the growth method, material properties, and applications for each material Includes chapters on the fundamentals of MOVPE and the key areas of equipment/safety, precursor chemicals, and growth monitoring Looks at important materials such as III-V and II-VI compounds, quantum dots, and nanowires Provides topical and wide-ranging coverage from well-known authors in the field Part of the Materials for Electronic and Optoelectronic Applications series Metalorganic Vapor Phase Epitaxy (MOVPE): Growth, Materials Properties and Applications is an excellent book for graduate students, researchers in academia and industry, as well as specialist courses at undergraduate/postgraduate level in the area of epitaxial growth (MOVPE/ MOCVD/ MBE).

Author: Guadalupe Valverde Aguilar
Publisher: BoD – Books on Demand
ISBN: 1789853338
Category : Technology & Engineering
Languages : en
Pages : 108

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The sol-gel method is a powerful route of synthesis used worldwide. It produces bulk, nano- and mesostructured sol-gel materials, which can encapsulate metallic and magnetic nanoparticles, non-linear azochromophores, perovskites, organic dyes, biological molecules, etc.. This can have interesting applications for catalysis, photocatalysis; drug delivery for treatment of neurodegenerative diseases such as cancer, Parkinson's and Azheimer's. In this book, valuable contributions related to novel materials synthesized by the sol-gel route are provided. The effect of the sol-gel method to synthesize these materials with potential properties is described, and how the variation of the parameters during the synthesis influences their design and allows to adjust their properties according to the desired application is discussed.

Author: Ghenadii Korotcenkov
Publisher: CRC Press
ISBN: 1482264595
Category : Science
Languages : en
Pages : 431

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Porous silicon is rapidly attracting increasing interest from various fields, including optoelectronics, microelectronics, photonics, medicine, sensor and energy technologies, chemistry, and biosensing. This nanostructured and biodegradable material has a range of unique properties that make it ideal for many applications. This book, the third of a

Author: Rada Savkina
Publisher: CRC Press
ISBN: 1000054314
Category : Science
Languages : en
Pages : 271

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Oxide-based materials and structures are becoming increasingly important in a wide range of practical fields including microelectronics, photonics, spintronics, power harvesting, and energy storage in addition to having environmental applications. This book provides readers with a review of the latest research and an overview of cutting-edge patents received in the field. It covers a wide range of materials, techniques, and approaches that will be of interest to both established and early-career scientists in nanoscience and nanotechnology, surface and material science, and bioscience and bioengineering in addition to graduate students in these areas. Features: Contains the latest research and developments in this exciting and emerging field Explores both the fundamentals and applications of the research Covers a wide range of materials, techniques, and approaches

Author: Zhe Chuan Feng
Publisher: Taylor & Francis
ISBN: 1439855714
Category : Technology & Engineering
Languages : en
Pages : 447

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Through their application in energy-efficient and environmentally friendly devices, zinc oxide (ZnO) and related classes of wide gap semiconductors, including GaN and SiC, are revolutionizing numerous areas, from lighting, energy conversion, photovoltaics, and communications to biotechnology, imaging, and medicine. With an emphasis on engineering a

Author: Faxian Xiu
Publisher:
ISBN:
Category : Molecular beam epitaxy
Languages : en
Pages : 396

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Author: Sotirios Baskoutas
Publisher: MDPI
ISBN: 3038973025
Category : Electronic books
Languages : en
Pages : 303

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This book is a printed edition of the Special Issue "Zinc Oxide Nanostructures: Synthesis and Characterization" that was published in Materials