Fabrication Of Zinc Nitride Thin Films Using Rf Magnetron Sputtering Deposition For Optoelectronic Applications

Zinc nitride thin films possess a small optical band gap with direct transition, low resistivity, high mobility and carrier concentration. Therefore, it may be suitable as an optoelectronic material for infrared sensors, smart windows and energy conversion devices. The objective of this work is to grow zinc nitride thin films using RF magnetron sputtering, understand its mechanical, optical, and electrical properties, and investigate its performance as light sensing devices. Synthesis and characterization of zinc nitride thin films has been investigated in this work. An RF magnetron sputtering deposition was employed to synthesize zinc nitride thin films using pure metal zinc target in either N2-Ar or N2-Ar-H2 mixtures. The microstructural, optical and electrical characterizations of the representative films were investigated with stylus profilometry, XRD, AFM, SEM, TEM, UV-VIS-NIR double beam spectrometry, and Hall effect measurement. The photoresponse of the zinc nitride photoconductors was also studied under the irradiation of white light and NIR light. The as-deposited zinc nitride thin films were relatively soft and densely packed with smooth surface. It possesses a narrow optical band gap in the NIR range with direct transition. The zinc nitride showed n-type conductivity with low resistivity and high carrier concentration. To study the RF discharge power effect, the zinc nitride thin films were synthesized at different discharge powers densities. With discharge power density increasing, the film deposition rate increased, and the zinc nitride films acquired better crystalline structure, smaller optical band gap and less oxygen contaminations. After thermal annealing at moderate temperatures in either air or O2, the annealed zinc nitride thin films were photoconductive under irradiation of both NIR light and white light. The largest photoresponse and fastest response times were measured at the room temperature for the zinc nitride thin films annealed at 300 degree in the air. Hydrogen inclusion can modify the electrical and optical properties of crystalline semiconductor films by introducing impurity donor states. The ZnNx:H films deposited in N2-Ar-H2 mixture acquired less oxygen contamination and higher relative nitrogen atom concentration than the ZnNx films deposited in N2-Ar mixture. The as-deposited ZnNx:H films showed a clear photonic behavior under white light irradiation, and the annealed ZnNx:H films exhibited a pronounced change in resistance under both white light and NIR light irradiation comparing to the annealed ZnNx films. This was the first time to report photoresponse of zinc nitride thin films fabricated by reactive sputtering method. The photoconductivity was gradually improved by optimization of deposition conditions, annealing conditions and film compositions.

This dissertation presents a study on the fabrication of zinc nitride and zinc oxy-nitride films, and related hetero-structures on glass, silicon and other substrates. The goals of this study include gaining fundamental understanding on the electrical and optical properties, the chemical-bonding states and the micro-structure of these materials and examining their potential for photovoltaic and other electronic and optoelectronic applications. Reactive radio-frequency (RF) magnetron sputtering was used as the deposition method, which potentially enables control of composition of the thin films, as well as fabrication of multilayer structures for the study of possible hetero-junctions between zinc nitride and zinc oxy-nitrides. Along with reactive sputtering, several other fabrication methods, such as thermal evaporation and solution (e.g. silver or carbon paste) painting, were used as auxiliaries where necessary. The characterization techniques employed include (i) x-ray based techniques (x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), energy dispersive x-ray spectroscopy (EDXS)), (ii) optical based methods (spectroscopic ellipsometry (SE), optical spectrophotometry, Raman spectroscopy), (iii) scanning electron microscopy (SEM), and (iv) electrical measurements (resistivity, Hall effect measurements, current-voltage and photovoltaic measurements). The cross-correlation between the deposition/post-deposition conditions and the physical properties of the films was investigated. The deposition conditions, such as the nitrogen (or oxygen) partial concentration in the sputtering gas mixture, substrate temperatures, total deposition pressure, as well as the post-deposition treatments such as thermal treatment and/or oxidation in ambient, were studied in detail. Zinc nitride, with a small fraction of "naturally" incorporated oxygen, is found to be a promising candidate for photovoltaic applications because of its optical and electrical properties. Also, the capability of property tuning for the zinc oxy-nitride material system was demonstrated by intentionally introducing varied amount oxygen into zinc nitride. In order to better understand the crystalline structure and the electronic band structure of these materials, first principle density functional theory (DFT) was used for computations of pure zinc nitride and the doping effects in it with both native elements (Zn, N) and copper family elements (Cu, Ag, Au) as possible p-type dopants. Atomic geometry, formation energy, as well as electronic structure of defects in zinc nitride were studied and a general consistency was observed between theoretically calculated and experimentally determined results. Defect density of states (DOS) suggest that among all three studied copper-family elements, copper is a good candidate for a p-type dopant. Technological insight and approaches to the fabrication of device-relevant structures were the other important outcomes of this work. Our studies showed that the fabrication of device-relevant ohmic contacts, rectifying metal-nitride junctions and p-n junctions was possible. Substantial photovoltaic action was observed in a single junction solar cell configuration that uses p-type zinc oxy-nitride as an absorber layer.

Novel materials, including zinc oxide (ZnO) and 2D transition metal dichalcogenides (TMDs), have been investigated in this dissertation for the realization of high-performance large-area integrated circuits. These novel materials may provide differential advantages over the established large-area thin film technology based on silicon, which has been extensively employed in applications such as large-area flat panel displays, high-speed active matrix thin film circuits, flexible and wearable electronics, etc. The dissertation begins with the discussion of high-performance plasma-enhanced atomic layer deposition (PEALD) of ZnO thin films and ZnO thin film transistors (TFTs) with a field effect mobility of ~ 10 to 20 cm2/Vs, which have been demonstrated. Offset-drain ZnO TFTs, which are able to withstand or switch voltage beyond 80 V, have also been demonstrated. These results shed light on the realization of large-area active-matrix circuits beyond the capabilities of the current display industry where high circuit speed or high operation voltage is required. To further improve the performance of ZnO-based electronics, many related materials, including doped ZnO, zinc nitride, and aluminum nitride, have been investigated. Doped ZnO has been proposed as the carrier injection layer that can improve the conductivity of metal-semiconductor contact in ZnO TFTs. Aluminum-doped ZnO thin films have been deposited using triisobutyl aluminum (TIBA) as the dopant precursor instead of trimethyl aluminum (TMA) in order to improve the uniformity of dopant distribution because TIBA has much lower vapor pressure than TMA. AZO thin films with resistivity ~ 10-2 cm have been achieved by PEALD. Besides, aluminum nitride and zinc nitride thin films have also been studied using PEALD. In addition to the showerhead PEALD system, a novel inductively coupled plasma ALD system has been designed and set up that provides RF power up to 500 W in order to generate a highly reactive nitrogen plasma source and enable the deposition of high-quality metal nitride at relatively low temperature. These metal nitride thin films may provide additional building blocks to enhance the speed and thermal stability of ZnO-based thin film devices and circuits.Owing to their excellent electrical and mechanical properties, 2D-TMD thin films have been studied for flexible electronics applications. High quality MoS2 and WS2 thin films have been achieved via mechanical exfoliation and chemical vapor deposition. To fabricate MoS2- and WS2-based TFTs, a 5-step device fabrication process has been developed, which is compatible to both the conventional rigid substrate and the ~ 4.8 nm thick solution-cast polyimide (PI) flexible substrate. The MoS2 and WS2 TFTs fabricated on PI substrate exhibit a field effect mobility of between 1 to 20 cm2/Vs, which is similar to that of those fabricated on rigid silicon substrate. More importantly, extraordinary mechanical strength and stability have been demonstrated for MoS2 and WS2 TFTs fabricated on PI substrate. A reasonably small degradation in device performance has been observed in these flexible 2D-TMD TFTs under static bending to the radius of ~ 2mm and after cyclic bending up to 100,000 cycles. Finally, attempts to create integratable 2D-TMD circuits have been demonstrated. To realize large-area 2D-TMD based circuits, growth of wafer-scale continuous WSe2 thin films has been demonstrated using metal organic chemical vapor deposition (MOCVD). Deposition has been achieved at as low as 400 C, which allows deposition on glass and polymeric substrate and enables the transfer-free fabrication of WSe2 TFTs and circuits on arbitrary platforms. Patterning and post-growth thickness modulation of continuous WSe2 thin film have been demonstrated using CF4 plasma and O2 plasma, whereby high-speed etching and nanometer-scale film thinning can be realized. With the capability of depositing and patterning wafer-scale WSe2 thin films, an array of p-channel WSe2 TFTs have been fabricated with a field effect mobility of ~0.01 cm2/Vs and an on-off ratio greater than 104.

This book provides a highly practical treatment of Glancing Angle Deposition (GLAD), a thin film fabrication technology optimized to produce precise nanostructures from a wide range of materials. GLAD provides an elegant method for fabricating arrays of nanoscale helices, chevrons, columns, and other porous thin film architectures using physical vapour deposition processes such as sputtering or evaporation. The book gathers existing procedures, methodologies, and experimental designs into a single, cohesive volume which will be useful both as a ready reference for those in the field and as a definitive guide for those entering it. It covers: Development and description of GLAD techniques for nanostructuring thin films Properties and characterization of nanohelices, nanoposts, and other porous films Design and engineering of optical GLAD films including fabrication and testing, and chiral films Post-deposition processing and integration to optimize film behaviour and structure Deposition systems and requirements for GLAD fabrication A patent survey, extensive relevant literature, and a survey of GLAD's wide range of material properties and diverse applications.

Oxynitride thin film technology is rapidly impacting a broad spectrum of applications, ranging from decorative functions (through optoelectronics) to corrosion resistance. Developing a better understanding of the relationships between deposition processes, structure and composition of the deposited films is critical to the continued evolution of these applications. This e-book provides valuable information about the process modeling, fabrication and characterization of metallic oxynitride-based thin films produced by reactive sputtering and some related deposition processes. Its contents are spread in twelve main and concise chapters through which the book thoroughly reviews the bases of oxynitride thin film technology and deposition processes, sputtering processes and the resulting behaviors of these oxynitride thin films. More importantly, the solutions for the growth of oxynitride technology are given in detail with an emphasis on some particular compounds. This is a valuable resource for academic learners studying materials science and industrial coaters, who are concerned not only about fundamental aspects of oxynitride synthesis, but also by their innate material characteristics.

This 3e, edited by Peter M. Martin, PNNL 2005 Inventor of the Year, is an extensive update of the many improvements in deposition technologies, mechanisms, and applications. This long-awaited revision includes updated and new chapters on atomic layer deposition, cathodic arc deposition, sculpted thin films, polymer thin films and emerging technologies. Extensive material was added throughout the book, especially in the areas concerned with plasma-assisted vapor deposition processes and metallurgical coating applications.

Recent years have witnessed the flourishing of numerous novel strategies based on the magnetron sputtering technique aimed at the advanced engineering of thin films, such as HiPIMS, combined vacuum processes, the implementation of complex precursor gases or the inclusion of particle guns in the reactor, among others. At the forefront of these approaches, investigations focused on nanostructured coatings appear today as one of the priorities in many scientific and technological communities: The science behind them appears in most of the cases as a "terra incognita", fascinating both the fundamentalist, who imagines new concepts, and the experimenter, who is able to create and study new films with as of yet unprecedented performances. These scientific and technological challenges, along with the existence of numerous scientific issues that have yet to be clarified in classical magnetron sputtering depositions (e.g., process control and stability, nanostructuration mechanisms, connection between film morphology and properties or upscaling procedures from the laboratory to industrial scales) have motivated us to edit a specialized volume containing the state-of-the art that put together these innovative fundamental and applied research topics. These include, but are not limited to: • Nanostructure-related properties; • Atomistic processes during film growth; • Process control, process stability, and in situ diagnostics; • Fundamentals and applications of HiPIMS; • Thin film nanostructuration phenomena; • Tribological, anticorrosion, and mechanical properties; • Combined procedures based on the magnetron sputtering technique; • Industrial applications; • Devices.