Download or read book Optimisation of ZnO Thin Films written by Saurabh Nagar and published by Springer. This book was released on 2017-05-22 with total page 101 pages. Available in PDF, EPUB and Kindle. Book excerpt: This monograph describes the different implantation mechanisms which can be used to achieve strong, reliable and stable p-type ZnO thin films. The results will prove useful in the field of optoelectronics in the UV region. This book will prove useful to research scholars and professionals working on doping and implantation of ZnO thin films and subsequently fabricating optoelectronic devices. The first chapter of the monograph emphasises the importance of ZnO in the field of optoelectronics for ultraviolet (UV) region and also discusses the material, electronic and optical properties of ZnO. The book then goes on to discuss the optimization of pulsed laser deposited (PLD) ZnO thin films in order to make successful p-type films. This can enable achievement of high optical output required for high-efficiency devices. The book also discusses a hydrogen implantation study on the optimized films to confirm whether the implantation leads to improvement in the optimized results.

Download or read book Optimization of the Process for Sol gel Derived ZnO written by Matthew Nagorski and published by . This book was released on 2021 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Transparent and conducting Al-doped ZnO thin films with c-axis-preferred orientation were prepared on glass substrate via sol-gel route. The physical, optical and electrical properties were investigated to determine an optimal withdrawal speed, aluminum source and treatment in order to obtain a smooth, dense, highly crystalline, conductive and transparent thin film with a high figure of merit for transparent conducting oxide applications. An optimal withdrawal speed was found to be 2.5 cm/min. Optimal aluminum source and concentration was found to be 0.5 at.% using aluminum chloride hexahydrate. An additional treatment in an N2 environment was found to be the best method to improve the electrical properties of the films while maintaining high crystallinity and transparency.

Download or read book The Optimization of Thin Film P CuO n ZnO Heterostructures for Use in Selective Gas Detection written by Christopher S. Dandeneau and published by . This book was released on 2009 with total page 266 pages. Available in PDF, EPUB and Kindle. Book excerpt: Since bulk p-CuO/n-ZnO heterocontacts were first proposed for gas detection, rapid development has taken place in improving the overall functionality of these structures. While bulk heterocontacts have been shown to exhibit desirable sensitivity and selectivity characteristics, these devices suffer from innate diffusion and ZnO/CuO connectivity drawbacks that limit their effective use. To address these issues, thin film p-CuO/n-ZnO heterostructures have been fabricated via wet chemical (sol-gel) processes so as to examine their potential use in reducing environments. Individual ZnO and CuO sol-gel processes have been developed with the goal of optimizing thin film porosity, crystallinity, and preferred orientation for enhanced gas sensing capability. Particular attention was given to the effects of solution chemistry and pyrolysis temperature on desired thin film properties. For ZnO, control over film microstructure was attained through fabrication modes based on the solvents 2-methoxyethanol (MOE) and dimethylformamide (DMF). Monoethanolamine (MEA) was employed as a chelating ligand in specific solutions. Optimum preferred orientation for DMF-based ZnO films was seen to exist at a solution chemistry of 5% water and a 1:1 molar ratio of Zn to MEA. An increase in the drying temperature yielded a monotonic decrease in the electrical resistivity of these films. For the MOE-based process, a lowering of the pyrolysis temperature led to an increase in ZnO film porosity. CuO thin films were deposited through a solution route based on isopropanol. Scanning electron microscopy (SEM) revealed the CuO films to possess a level of porosity much higher than that seen in the ZnO films. Thin film p-CuO/n-ZnO heterostructures were fabricated in two configurations; ZnO on CuO (ZnO/CuO) and CuO on ZnO (CuO/ZnO). The results of current-voltage (I-V) tests showed the CuO/ZnO structures to display enhanced sensing characteristics to 4000 ppm hydrogen when compared to the ZnO/CuO structures. This finding was attributed to the inherently high porosity of the top CuO layer which in turn allowed for improved gas diffusion to the heterostructure interface. The phase equilibrium between CuO and ZnO exhibits limited solubility. As such, a novel microstructure formed by combining CuO and ZnO precursors has been explored with the expectation that the films will phase separate. For the deposited films, a variance in both the annealing temperature and time was found to yield a microstructure comprised of individual ZnO and CuO grains. The co-existence of these two structures was confirmed through Energy Dispersive Spectroscopy (EDS). It is expected that the high level of connectivity between the ZnO and CuO along with negligible barriers to gas diffusion will lead to superior sensing characteristics.

Download or read book Zinc Oxide Bulk Thin Films and Nanostructures written by Chennupati Jagadish and published by Elsevier. This book was released on 2011-10-10 with total page 600 pages. Available in PDF, EPUB and Kindle. Book excerpt: With an in-depth exploration of the following topics, this book covers the broad uses of zinc oxide within the fields of materials science and engineering: - Recent advances in bulk , thin film and nanowire growth of ZnO (including MBE, MOCVD and PLD), - The characterization of the resulting material (including the related ternary systems ZgMgO and ZnCdO), - Improvements in device processing modules (including ion implantation for doping and isolation ,Ohmic and Schottky contacts , wet and dry etching), - The role of impurities and defects on materials properties - Applications of ZnO in UV light emitters/detectors, gas, biological and chemical-sensing, transparent electronics, spintronics and thin film

Download or read book Processing and Characterization of P Type Doped Zinc Oxide Thin Films written by Michelle Anne Myers and published by . This book was released on 2013 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Applications of zinc oxide (ZnO) for optoelectronic devices, including light emitting diodes, semiconductor lasers, and solar cells have not yet been realized due to the lack of high-quality p-type ZnO. In the research presented herein, pulsed laser deposition is employed to grow Ag-doped ZnO thin films, which are characterized in an attempt to understand the ability of Ag to act as a p-type dopant. By correlating the effects of the substrate temperature, oxygen pressure, and laser energy on the electrical and microstructural properties of Ag-doped ZnO films grown on c-cut sapphire substrates, p-type conductivity is achieved under elevated substrate temperatures. Characteristic stacking fault features have been continuously observed by transmission electron microscopy in all of the p-type films. Photoluminescence studies on n-type and p-type Ag-doped ZnO thin films demonstrate the role of stacking faults in determining the conductivity of the films. Exciton emission attributed to basal plane stacking faults suggests that the acceptor impurities are localized nearby the stacking faults in the n-type films. The photoluminescence investigation provides a correlation between microstructural characteristics and electrical properties of Ag- doped ZnO thin films; a link that enables further understanding of the doping nature of Ag impurities in ZnO. Under optimized deposition conditions, various substrates are investigated as potential candidates for ZnO thin film growth, including r -cut sapphire, quartz, and amorphous glass. Electrical results indicated that despite narrow conditions for obtaining p-type conductivity at a given substrate temperature, flexibility in substrate choice enables improved electrical properties. In parallel, N+-ion implantation at elevated temperatures is explored as an alternative approach to achieve p-type ZnO. The ion implantation fluence and temperature have been optimized to achieve p-type conductivity. Transmission electron microscopy reveals that characteristic stacking fault features are present throughout the p-type films, however in n-type N-doped films high-density defect clusters are observed. These results suggest that the temperature under which ion implantation is performed plays a critical role in determining the amount of dynamic defect re- combination that can take place, as well as defect cluster formation processes. Ion implantation at elevated temperatures is shown to be an effective method to introduce increased concentrations of p-type N dopants while reducing the amount of stable post-implantation disorder. Finally, the fabrication and properties of p-type Ag-doped ZnO/n-type ZnO and p-type N-doped ZnO/n-type ZnO thin film junctions were reported. For the N-doped sample, a rectifying behavior was observed in the I-V curve, consistent with N-doped ZnO being p-type and forming a p-n junction. The turn-on voltage of the device was -2.3 V under forward bias. The Ag-doped samples did not result in rectifying behavior as a result of conversion of the p-type layer to n-type behavior under the n- type layer deposition conditions. The systematic studies in this dissertation provide possible routes to grow p-type Ag-doped ZnO films and in-situ thermal activation of N-implanted dopant ions, to overcome the growth temperature limits, and to push one step closer to the future integration of ZnO-based devices. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/149354

Download or read book ZnO Thin film Optimization Towards the Fabrication of High frequency Ultrasound Transducers written by Ihab Sinno and published by . This book was released on 2017 with total page 261 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book ZnO Thin Films written by Paolo Mele and published by . This book was released on 2019 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Zinc oxide (ZnO) is an n-type semiconductor with versatile applications such as optical devices in ultraviolet region, piezoelectric transducers, transparent electrode for solar cells and gas sensors. This book "ZnO Thin Films: Properties, Performance and Applications" gives a deep insight in the intriguing science of zinc oxide thin films. It is devoted to cover the most recent advances and reviews the state of the art of ZnO thin films applications involving energy harvesting, microelectronics, magnetic devices, photocatalysis, photovoltaics, optics, thermoelectricity, piezoelectricity, electrochemistry, temperature sensing. It serves as a fundamental information source on the techniques and methodologies involved in zinc oxide thin films growth, characterization, post-deposition plasma treatments and device processing. This book will be invaluable to the experts to consolidate their knowledge and provide insight and inspiration to beginners wishing to learn about zinc oxide thin films.

Download or read book Optimization of the Fabrication Condition of RF Sputtered ZnO Thin Film Transistors with High k HfO2 Gate Dielectric written by Prem Thapaliya and published by . This book was released on 2016 with total page 182 pages. Available in PDF, EPUB and Kindle. Book excerpt: Conventional amorphous silicon based thin film transistors have been the most widely used ones for flat panel display application during the last two decades. However, the low mobility of less than 1 cm2/Vs and light induced instability of the amorphous silicon based thin film transistor make them unsuitable for high resolution displays. Oxide based thin film transistors have attracted a great deal of interest as an alternative to conventional amorphous silicon based thin film transistors for high resolution display applications. In particular, ZnO has gained considerable interest for the next generation transparent and flexible display due to its wide band gap of 3.37 eV, high electron mobility and low temperature deposition forming good quality of polycrystalline film even at room temperature. Consequently all the aforementioned features of ZnO make them promising channel material for the flexible and transparent TFTs. The electrical characteristics of ZnO based TFTs is greatly affected by the deposition condition and hence crystalline quality of channel layer, thickness of channel layer and quality of interface between the gate dielectric and the channel layer. Therefore, the deposition temperature and the thickness of the ZnO channel needs to be optimized in order to achieve high performance ZnO TFTs. Moreover, the quality of interface between the ZnO channel layer and the gate dielectric is of vital importance to improve the performance of the TFTs. In this dissertation, we have fabricated and characterized RF sputtered ZnO based thin film transistor using high-k HfO2 gate dielectric. The transparent ZnO TFTs was realized using FTO as a transparent gate electrode as opposed to commonly used ITO gate electrode. It was found that TFTs fabricated using the FTO gate electrode showed lower mobility and on/off ratio compared to the TFTs with Ru as a gate on the Si substrate. This deterioration of TFTs performance with the use of FTO gate electrode was attributed to the degradation of HfO2 gate dielectric due to the diffusion of fluorine from the FTO into the HfO2 during its deposition at 300 °C. In order to minimize the interface trap density at the interface between the ZnO and HfO2, an interfacial layer of MgO with different thickness was investigated. It was found that 10 nm MgO is an optimum thickness that can reduce the interface trap density by almost one order of magnitude and hence exhibit the best TFTs performance with field effect mobility, threshold voltage, on/off ratio and subthreshold swing to be 0.3 cm2/V.s, 3.7 V , 106 and 1.35 V/decade respectively. The decrease in the interface trap density with the interfacial layer was attributed to the reduction of defects in the ZnO by the excess oxygen ions of MgO. Furthermore, the ZnO channel layer was deposited at different temperature including room temperature, 50 °C, 100 °C and 200 °C, to determine the optimum deposition temperature that can achieve high performance ZnO TFTs. It was found that ZnO deposited at 50 °C showed the best TFT performance with field effect mobility, threshold voltage, on off ratio and subthreshold swing 1.12 cm2/V.s, 5.8 V, 1.4×105, 1.35 V/decade respectively. The improvement in the performance of the TFTs device with 50 °C ZnO was attributed to the low surface roughness of ZnO film, increased grain size and good polycrystalline quality which was confirmed with the help of XRD, AFM and SEM measurement of ZnO thin film deposited at different temperature. Likewise, once the optimum deposition temperature of ZnO was determined, the effect of ZnO thickness was investigated by depositing the ZnO with different thickness including 30 nm, 50 nm, 70 nm and 100 nm while maintaining the deposition temperature of ZnO to be at 50 °C. It was found that the TFTs device with 50 nm exhibit the superior performance over the other thicknesses of ZnO which was ascribed to the improved polycrystalline quality, low surface roughness of the 50 nm ZnO thin film.

Download or read book Doping in Zinc Oxide Thin Films written by Zheng Yang and published by . This book was released on 2009 with total page 149 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Synthesis of Atomic Layer Deposition Zinc Oxide and Thin Film Materials Optimization for Ultraviolet Photodector Applications written by Kandabara Nouhoum Tapily and published by . This book was released on 2011 with total page 336 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book ZnO Thin Films for Optoelectronic Applications written by Prasada Rao Talakonda and published by LAP Lambert Academic Publishing. This book was released on 2013 with total page 220 pages. Available in PDF, EPUB and Kindle. Book excerpt: Zinc oxide (ZnO) thin films have good electro-optical properties suitable for opto-electronic applications. The present study explains the deposition and characterization of n-type and p-type ZnO thin films by spray pyrolysis. The films were characterized by different methods to understand their structural, optical and electrical properties. Gallium was chosen as the impurity dopant in ZnO films to improve the electrical properties. The electrical conductivity, carrier concentration and mobility of Ga doped ZnO (GZO) films were highly improved in comparison to undoped ZnO films. The GZO films showed good optical transmittance in the visible region. The electrical and optical results suggest that the GZO films are suitable to use as a TCO in optoelectronic industries. The p-type ZnO thin films were successesfully realized using dual acceptor method. The Hall measurements and room temperature photolumiscence results were supported p-type nature of (Li, N): ZnO thin films.

Download or read book Optoelectronics Devices Based on Zinc Oxide Thin Films and Nanostructures written by Sheng Chu and published by . This book was released on 2011 with total page 107 pages. Available in PDF, EPUB and Kindle. Book excerpt: Optoelectronics devices based on ZnO thin films and nanostructures are discussed in this dissertation. A ZnO homojunction LED was demonstrated. Sb-doped p-type ZnO and Ga-doped n-type ZnO on Si (100) substrate were used for the LED device. After achieving ohmic contacts on both types of ZnO, the device showed rectifying current-voltage (I-V) characteristics. Under forward bias, the device successfully showed ultraviolet emissions. The emission properties were analyzed and the emission was confirmed to come from ZnO near band edge emissions. Further analysis showed that the emission mainly comes from the p-type layer of the device. A ZnO ultraviolet laser diode was fabricated and demonstrated. The device consists of Sb-doped p-type ZnO layer and Ga-doped n-type ZnO layer. In between p-layer and n-layer, a thin MgZnO/ZnO/MgZnO quantum well structure was inserted. In this device, random lasing mechanism plays an important role. When the diode was biased, the generation of light was enhanced by the carrier localization effect from the quantum well. The light was scattered between the ZnO random grain boundaries. Since the scattering effect can be so intense that some of the light can return to its original place to form close travel loop, as "random laser cavity". As long as the gain can overcome loss from scattering and material loss, lasing action can be demonstrated. An improved ZnO LED device was grown and characterized. The device grown on c-plane sapphire substrate can favor ultimate device applications due to the improved crystal quality of ZnO and the possibility of getting single crystallinity. A double heterostructure (MgZnO/ZnO/MgZnO) was also inserted in between p-layer and n-layer of the device to enhance the light output. The device showed much enhanced output power of 457 nW, which is two orders stronger than the LED fabricated on Si substrate. The optimization of high quality ZnO thin film on c-plane sapphire substrate was discussed. The devices in chapter two, three and four utilized Si or sapphire substrate, and are all in polycrystalline nature. To solve this problem and get the basis of high output power LEDs and lasers, single crystalline, two dimensional surface ZnO thin films were grown in chapter five. MgO/ZnO double buffer layers were used to accommodate the lattice mismatch. MgO thickness was found to be very important in achieving good ZnO thin film. An optimized growth also yields low background electron concentration and high mobility, which can enable future high quality p-type ZnO engineering. Our research was also expanded from ZnO thin films to ZnO nanostructures. The purpose of chapter six is to demonstrate a ZnO nanowire laser. ZnO nanowires are an excellent cavity and itself is a great gain material. We expanded Sb-doped p-type ZnO from thin films to ZnO nanowires. A p-type ZnO nanowire/n-type ZnO thin film p-n junction was achieved. The device showed lasing action when injection current was larger than ~50 mA. The lasing mechanism and gain/feedback were also discussed in detail.

Download or read book Optimization of Dye sensitized Solar Cells Using Thin film ZnO and TiO2 Semiconducting Materials Fabricated by Electrophoretic Deposition written by Yukihiro Hara and published by . This book was released on 2011 with total page 137 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Toward the Optimization of Low temperature Solution based Synthesis of ZnO Nanostructures for Device Applications written by Hatim Alnoor and published by Linköping University Electronic Press. This book was released on 2017-10-06 with total page 67 pages. Available in PDF, EPUB and Kindle. Book excerpt: One-dimensional (1D) nanostructures (NSs) of Zinc Oxide (ZnO) such as nanorods (NRs) have recently attracted considerable research attention due to their potential for the development of optoelectronic devices such as ultraviolet (UV) photodetectors and light-emitting diodes (LEDs). The potential of ZnO NRs in all these applications, however, would require synthesis of high crystal quality ZnO NRs with precise control over the optical and electronic properties. It is known that the optical and electronic properties of ZnO NRs are mostly influenced by the presence of native (intrinsic) and impurities (extrinsic) defects. Therefore, understanding the nature of these intrinsic and extrinsic defects and their spatial distribution is critical for optimizing the optical and electronic properties of ZnO NRs. However, identifying the origin of such defects is a complicated matter, especially for NSs, where the information on anisotropy is usually lost due to the lack of coherent orientation. Thus, the aim of this thesis is towards the optimization of the lowtemperature solution-based synthesis of ZnO NRs for device applications. In this connection, we first started with investigating the effect of the precursor solution stirring durations on the deep level defects concentration and their spatial distribution along the ZnO NRs. Then, by choosing the optimal stirring time, we studied the influence of ZnO seeding layer precursor’s types, and its molar ratios on the density of interface defects. The findings of these investigations were used to demonstrate ZnO NRs-based heterojunction LEDs. The ability to tune the point defects along the NRs enabled us further to incorporate cobalt (Co) ions into the ZnO NRs crystal lattice, where these ions could occupy the vacancies or interstitial defects through substitutional or interstitial doping. Following this, high crystal quality vertically welloriented ZnO NRs have been demonstrated by incorporating a small amount of Co into the ZnO crystal lattice. Finally, the influence of Co ions incorporation on the reduction of core-defects (CDs) in ZnO NRs was systematically examined using electron paramagnetic resonance (EPR).

Download or read book SYNTHESIS AND CHARACTERIZATION OF TRANSPARENT CONDUCTIVE ZINC OXIDE THIN FILMS BY SOL GEL SPIN COATING METHOD written by David Winarski and published by . This book was released on 2015 with total page 87 pages. Available in PDF, EPUB and Kindle. Book excerpt: Zinc oxide has been given much attention recently as it is promising for various semiconductor device applications. ZnO has a direct band gap of 3.3 eV, high exciton binding energy of 60 meV and can exist in various bulk powder and thin film forms for different applications. ZnO is naturally n-type with various structural defects, which sparks further investigation into the material properties. Although there are many potential applications for this ZnO, an overall lack of understand and control of intrinsic defects has proven difficult to obtain consistent, repeatable results. This work studies both synthesis and characterization of zinc oxide in an effort to produce high quality transparent conductive oxides. The sol-gel spin coating method was used to obtain highly transparent ZnO thin films with high UV absorbance. This research develops a new more consistent method for synthesis of these thin films, providing insight for maintaining quality control for each step in the procedure. A sol-gel spin coating technique is optimized, yielding highly transparent polycrystalline ZnO thin films with tunable electrical properties. Annealing treatment in hydrogen and zinc atmospheres is researched in an effort to increase electrical conductivity and better understand intrinsic properties of the material. These treatment have shown significant effects on the properties of ZnO. Characterization of doped and undoped ZnO synthesized by the sol-gel spin coating method was carried out using scanning electron microscopy, UV-Visible range absorbance, X-ray diffraction, and the Hall Effect. Treatment in hydrogen shows an overall decrease in the number of crystal phases and visible absorbance while zinc seems to have the opposite effect. The Hall Effect has shown that both annealing environments increase the n-type conductivity, yielding a ZnO thin film with a carrier concentration as high as 3.001 × 1021 cm-3.

Download or read book Transparent Conductive Zinc Oxide written by Klaus Ellmer and published by Springer Science & Business Media. This book was released on 2007-12-29 with total page 453 pages. Available in PDF, EPUB and Kindle. Book excerpt: Zinc oxide (ZnO) belongs to the class of transparent conducting oxides that can be used as transparent electrodes in electronic devices or heated windows. In this book the material properties of, the deposition technologies for, and applications of zinc oxide in thin film solar cells are described in a comprehensive manner. Structural, morphological, optical and electronic properties of ZnO are treated in this review.

Download or read book Electrical Characterization of ZnO thin films grown by molecular beam epitaxy written by Vladimir Petukhov and published by Cuvillier Verlag. This book was released on 2012-04-25 with total page 112 pages. Available in PDF, EPUB and Kindle. Book excerpt: For the electronic and optoelectronic device realization a precise control of the electrical properties in the utilized material is a very important issue. Doping profiles in realized p-njunctions influence the functionality of the devices. The morphological and crystal properties of a device material directly influence the electrical ones. Dislocations present in a region of p-n-junctions can short circuit them leading to malfunctions. Too rough surfaces during epitaxial growth could lead to inhomogeneities in a single or multiple quantum wells and superlattices. The main goal of the present work was to provide the basis for a reliable p-type doping of ZnO grown by molecular beam epitaxy. Firstly, the well established heteroepitaxial growth on c-sapphire substrates has been employed. Based on the theoretical and experimental works, suggesting nitrogen to be the impurity that builds the most shallow acceptor level in ZnO comparing to other group-V elements, it has been implied as a dopant. To generate reactive nitrogen atoms an rf-plasma source has been utilized in the MBE process. The resulting samples have been characterized by such methods as AFM, XRD, TEM, PL spectroscopy, temperature domain Hall measurements (TDHM) and ECV-profiling. First results of TDHM have shown that even in undoped samples the temperature dependencies of the electron mobility and carrier concentration have regions which are difficult to interpret. It is necessary to fit them with theoretical curves in order to extract the correct values. This task has proven to be very difficult. The complicated character of the dependencies has been explained in terms of the multilayer conduction model dividing a layer in thin interfacial region with mobility and carrier concentration μ1 and n1 respectivly and bulk region with a higher mobility μ2 and lower carrier concentration n2. The electrical transport in the bulk region has been modeled in terms of the general scattering theory in polar semiconductors. Such scattering mechanisms as scattering on polar-optical phonons, piezoelectric phonons, acoustic deformation potential, strain induced fields, dislocations, ionized and neutral impurities have been taken into account. Two cases have been considered to model transport in the interfacial region: 1) transport takes place in the conduction band of a highly doped degenerate semiconductor; 2) transport takes place in the impurity band formed by intermediate concentration of impurities and in conduction band in parallel. In the second case transport at the interface in conduction band has been neglected in the region of the low temperatures due to the impurities freeze-out and carrier concentration has been taken temperature independent like in the first case. To investigate experimentally the transport character in these two regions independently a mobility-spectrum analysis has been conducted. Theoretical results utilizing the two models have been compared with experimentally extracted mobility and carrier concentration in the interfacial region. It has been concluded that the concentration of donors in the layers is not high enough for the impurity band to merge with the conduction band and the second model is more consistent. The theoretically acquired donor concentration profiles have been compared with ECV-profiles. The agreement is very good. Simulations have revealed a shallow donor state with the ionization energy of approximately 45 meV . In the literature, this donor state in ZnO is attributed to hydrogen. However, due to the high diffusion mobility of hydrogen in ZnO, an annealing process would obviously decrease the carrier concentration in the samples which has not been the case. It has been suggested that the main donor centers are the electrically active crystal point defects generated by dislocations. Layers doped with nitrogen have been grown at very low temperatures (≈ 200°C) and at temperatures ranging from 400°C to 500°C, which are optimal for the epitaxial growth of ZnO. The samples grown at low temperatures are single crystalline with mosaic structure. In both cases, the introduction of the dopant increased the carrier concentration. This has been accounted for a bad crystal quality resulting in the inhomogeneous incorporation of nitrogen and for high background donor concentration due to the high dislocations densities. Additionally, the incorporation of acceptor centers shifts the Fermi-level increasing the formation probability of the compensating point defects. The analysis of TDHM showed an inconsistency of the one donor level model in the case of nitrogen doped samples. This fact and the decrease in the carrier concentration after annealing at 800°C for 30 minutes in ambient air can be explained by nitrogen forming donor-like defect complexes. In an attempt to improve the crystal quality of the heteroepitaxial layers, 15 periods of a ZnO/Zn0.6Mg0.4O superlattice structure have been inserted between the conventional double HT-MgO/LT-ZnO buffer and a main HT-ZnO layer. TDHM has revealed a very high mobility close to the values measured in a bulk ZnO for the temperature range of 20 - 300 K. However, TEM investigations of the samples have not indicated any decrease in the dislocation density comparing with the similar samples without a superlattice. Such a high mobility has been attributed to an electron transport in the superlattice structure. Heteroepitaxial growth of high quality ZnO-layers has proven to be challenging leaving the homoepitaxial growth as the only possibility to obtain the epitaxial layers with the best structural and electrical properties. The hydrothermally grown bulk ZnO substrates from two supplying companies, CrysTec and TokyoDenpa, have been employed for homoepitaxy. The substrates from CrysTec have not been epi-ready. Although AFM images reveal very flat surface, this has been damaged by the process of the chemomechanical polishing. This damaged layer must be removed. This has been achieved by the thermal annealing for 3 hours at 1050°C in ambient air. The thermally treated surfaces resulted in atomically flat terraces. XRD measurements have indicated an improvement of the crystal quality after annealing. The resistivity of the bulk substrates decreased after the thermal treatment due to out-diffusion of the compensating Li atoms letting Al, Ga and In atoms to contribute to conduction. After the longer annealing processes the etch-pits have been discovered on O-polar faces. The same features could be achieved by the chemical etching in a nitric acid on Zn-polar faces. The density of the threading dislocations on both polar faces for both types of substrates calculated by the etch-pit density investigation is about 105 1/cm2. Further the thermally treated substrates with atomically flat terraces have been utilized for homoepitaxy. The differences in growth kinetics during the molecular beam epitaxy on such substrates with the improved surface quality depending on their polarity have been investigated by RHEED measurements. The growth on a Zn-polar face has a 3D-character independently on a supplier. Morphologies of the resulting O- and Zn-polar layers have shown to be different. This has been explained by the presence of dangling bonds on Opolar face and thus, shorter diffusion time of the impinging Zn atoms on the surface. XRD and TEM measurements have shown a perfect crystal quality of the overgrown layers. The PL spectra of homoepitaxial layers are governed by the donor impurities diffused from the substrates. Considering the SIMS measurements of homoepitaxial layers found in the literature it has been concluded that the diffusion of donors in the layers grown on Zn-polar faces takes less effect then for the O-polar films. This conclusion has enforced the utilization of Zn-polar substrates supplied by CrysTec for the experiments with nitrogen doping of ZnO because of their affordable price. The electrical properties measured by ECV-profiling in series of homoepitaxial layers with varied growth parameters have shown an increase of the carrier concentration with the nitrogen incorporation. In addition, it has also been shown that the resulting electrical properties near the interface are governed mostly by the initial properties of the substrates. With increasing thickness of the layers carrier concentration saturated to the values of around 1016 1/cm3. The recent successful realization of the p-type MgZnO layers on TokyoDenpa substrates by researchers from Japan suggests switching to the p-type doped alloys because the above discussed results indicate that p-type doping with nitrogen of a pure ZnO is very difficult or even impossible. This is due to a rather fundamental reason: the formation of the compensating donor centers with the incorporation of acceptor atoms. As the first step in the future works, it is obvious to try to reproduce the results of the ZnMgO p-type doping with nitrogen employing growth on ZnO substrates.