There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Conductometric semiconducting metal oxide gas sensors have been widely used and investigated in the detection of gases. Investigations have indicated that the gas sensing process is strongly related to surface reactions, so one of the important parameters of gas sensors, the sensitivity of the metal oxide based materials, will change with the factors influencing the surface reactions, such as chemical components, surface-modification and microstructures of sensing layers, temperature and humidity. In this brief review, attention will be focused on changes of sensitivity of conductometric semiconducting metal oxide gas sensors due to the five factors mentioned above.

Metal oxide gas sensors: Sensitivity and influencing factors

Metal oxide-based resistive-type gas sensors are solid-state devices which are widely used in a number of applications from health and safety to energy efficiency and emission control. Nanomaterials such as nanowires, nanorods, and nanoparticles have dominated the research focus in this field due to their large number of surface sites facilitating surface reactions. Previous studies have shown that incorporating two or more metal oxides to form a heterojunction interface can have drastic effects on gas sensor performance, especially the selectivity. Recently, these effects have been amplified by designing heterojunctions on the nano-scale. These designs have evolved from mixed commercial powders and bi-layer films to finely-tuned core–shell and hierarchical brush-like nanocomposites. This review details the various morphological classes currently available for nanostructured metal-oxide based heterojunctions and then presents the dominant electronic and chemical mechanisms that influence the performance of these materials as resistive-type gas sensors. Mechanisms explored include p–n and n–n potential barrier manipulation, n–p–n response type inversions, spill-over effects, synergistic catalytic behavior, and microstructure enhancement. Tables are presented summarizing these works specifically for SnO2, ZnO, TiO2, In2O3, Fe2O3, MoO3, Co3O4, and CdO-based nanocomposites. Recent developments are highlighted and likely future trends are explored.

Nanoscale metal oxide-based heterojunctions for gas sensing: A review

This review paper encompasses a detailed study of semiconductor metal oxide (SMO) gas sensors. It provides for a detailed comparison of SMO gas sensors with other gas sensors, especially for ammonia gas sensing. Different parameters which affect the performance (sensitivity, selectivity and stability) of SMO gas sensors are discussed here under. This paper also gives an insight about the dopant or impurity induced variations in the SMO materials used for gas sensing. It is concluded that dopants enhance the properties of SMOs for gas sensing applications by changing their microstructure and morphology, activation energy, electronic structure or band gap of the metal oxides. In some cases, dopants create defects in SMOs by generating oxygen vacancy or by forming solid solutions. These defects enhance the gas sensing properties. Different nanostructures (nanowires, nanotubes, heterojunctions), other than nanopowders have also been studied in this review. At the end, examples of SMOs are given to illustrate the potential use of different SMO materials for gas sensing.

Semiconductor metal oxide gas sensors: A review

Sensor technology is one of the most important key technologies of the future with a constantly increasing number of applications, both in the industrial and in the private sectors. More and more gas sensors are used for the control of technical processes, in environment monitoring, healthcare, and automobiles. Consequently, the development of fast and sensitive gas sensors with small cross sensitivity is the subject of intense research, propelled by strategies based on nanoscience and -technology. Established systems can be improved and novel sensor concepts based on bottom-up approaches show that the sensor properties can be controlled by molecular design. This Review highlights the recent developments and reflects the impact of nanoscience on sensor technology.

Metal and Metal Oxide Nanoparticles in Chemiresistors: Does the Nanoscale Matter?

In the recent past a great deal of research efforts were directed toward the development of miniaturized gas-sensing devices, particularly for toxic gas detection and for pollution monitoring. Though various techniques are available for gas detection, solid state metal oxides offer a wide spectrum of materials and their sensitivities for different gaseous species, making it a better choice over other options. In this article a critical parameter analysis of different metal oxides that are known to be sensitive to various gaseous species are thoroughly examined. This includes phase of the oxide, sensing gaseous species, operating temperature range, and physical form of the material for the development of integrated gas sensors. The oxides that are covered in this study include oxides of aluminum, bismuth, cadmium, cerium, chromium, cobalt, copper, gallium, indium, iron, manganese, molybdenum, nickel, niobium, ruthenium, tantalum, tin, titanium, tungsten, vanadium, zinc, zirconium, and the mixed or...

Oxide Materials for Development of Integrated Gas Sensors—A Comprehensive Review

Metal Oxide Nanostructures as Gas Sensing Devices

Preface About the Author Introduction Silicon: The Semiconductor Why Single Crystals Revolution in Integrated Circuit Fabrication Technology and the Art of Device Miniaturization Use of Silicon as a Semiconductor Silicon Devices for Boolean Applications Integration of Silicon Devices and the Art of Circuit Miniaturization MOS and CMOS Devices for Digital Applications LSI, VLSI, and ULSI Circuits and Applications Silicon for MEMS Applications Summary References Silicon: The Key Material for Integrated Circuit Fabrication Technology Introduction Preparation of Raw Silicon Material Metallurgical-Grade Silicon Purification of Metallurgical-Grade Silicon Ultra-High Pure Silicon for Electronics Application Polycrystalline Silicon Feed for Crystal Growth Summary References Importance of Single Crystals for Integrated Circuit Fabrication Introduction Crystal Structures Different Crystal Structures in Nature Cubic Structures Diamond Crystal Structure Silicon Crystal Structure Silicon Crystals and Atomic Packing Factors Crystal Order and Perfection Crystal Orientations and Planes Influence of Dopants and Impurities in Silicon Crystals Summary References Different Techniques for Growing Single-Crystal Silicon Introduction Bridgman Crystal Growth Technique Czochralski Crystal Growth/Pulling Technique Crucible Choice for Molten Silicon Chamber Temperature Profile Seed Selection for Crystal Pulling Environmental and Ambient Control in the Crystal Chamber Crystal Pull Rate and Seed/Crucible Rotation Dopant Addition for Growing Doped Crystals Methods for Continuous Czochralski Crystal Growth Impurity Segregation between Liquid and Grown Silicon Crystals Crystal Growth Striations Use of a Magnetic Field in the Czochralski Growth Technique Large-Area Silicon Crystals for VLSI and ULSI Applications Post-Growth Thermal Gradient and Crystal Cooling after Pull-Out Float-Zone Crystal Growth Technique Seed Selection Environment and Chamber Ambient Control Heating Mechanisms and RF Coil Shape Crystal Growth Rate and Seed Rotation Dopant Distribution in Growing Crystals Impurity Segregation between Liquid and Grown Silicon Crystals Use of Magnetic Field for Float-Zone Growth Large Area Silicon Crystals and Limitations of Shape and Size Thermal Gradient and Post-Growth Crystal Cooling Zone Refining of Single-Crystal Silicon Other Silicon Crystalline Structures and Growth Techniques Silicon Ribbons Silicon Sheets Silicon Whiskers and Fibers Silicon in Circular and Spherical Shapes Silicon Hollow Tubes Casting of Polycrystalline Silicon for Photovoltaic Applications Summary References From Silicon Ingots to Silicon Wafers Introduction Radial Resistivity Measurements Boule Formation, Identification of Crystal Orientation, and Flats Ingot Slicing Mechanical Lapping of Wafer Slices Edge Profiling of Slices Chemical Etching and Mechanical Damage Removal Chemimechanical Polishing for Planar Wafers Surface Roughness and Overall Wafer Topography Megasonic Cleaning Final Cleaning and Inspection Summary References Evaluation of Silicon Wafers Introduction Acoustic Laser Probing Technique Atomic-Force Microscope Studies on Surfaces Auger Electron Spectroscopic Studies Chemical Staining and Etching Techniques Contactless Characterization Deep-Level Transient Spectroscopy Defect Decoration by Metals Electron Beam and High-Energy Electron Diffraction Studies Flame Emission Spectrometry Four-Point Probe Technique for Resistivity Measurement and Mapping Fourier Transform Infrared Spectroscopy Measurements for Impurity Identification Gas Fusion Analysis Hall Mobility Mass Spectra Analysis Minority Carrier Diffusion Length/Lifetime/Surface Photovoltage Optical Methods for Impurity Evaluation Photoluminescence Method for Determining Impurity Concentrations Gamma-Ray Diffractometry Scanning Electron Microscopy for Defect Analysis Scanning Optical Microscope Secondary Ion Mass Spectrometer for Impurity Distribution Spreading Resistance and Two-Point Probe Measurement Technique Stress Measurements Transmission Electron Microscopy van der Pauw Resistivity Measurement Technique for Irregular-Shaped Wafers X-Ray Technique for Crystal Perfection and Dislocation Density Summary References Resistivity and Impurity Concentration Mapping of Silicon Wafers Introduction Electrically Active and Inactive Impurities Surface Mapping and Concentration Contours Surface Roughness Mapping on a Complete Wafer Summary References Impurities in Silicon Wafers Effect of Intentional and Unintentional Impurities and Their Influence on Silicon Devices Intentional Dopant Impurities in Silicon Wafers Aluminum Antimony Arsenic Boron Gallium Phosphorus Unintentional Dopant Impurities in Silicon Wafers Carbon Chromium Copper Germanium Gold Helium Hydrogen Iron Nickel Nitrogen Oxygen Tin Other Metallic Impurities Summary References Defects in Silicon Wafers Introduction Impact of Defects in Silicon Devices and Structures Point Defects and Vacancies Line Defects Bulk Defects and Voids Dislocations and Screw Dislocations Swirl Defects Stacking Faults Precipitations Surface Pits/Crystal-Originated Particles Grown Vacancies and Defects Thermal Donors Slips, Cracks, and Shape Irregularities Stress, Bowing, and Warpage Summary References Silicon Wafer Preparation for VLSI and ULSI Processing Introduction Purity of Chemicals Used for Silicon Processing Degreasing of Silicon Wafers Removal of Metallic and Other Impurities Gettering of Metallic Impurities Denuding of Silicon Wafers Neutron Irradiation Argon Annealing of Wafers Hydrogen Annealing of Wafers Final Cleaning, Rinsing, and Wafer Drying Summary References Packing of Silicon Wafers Packing of Fully Processed Blank Silicon Wafers Storage of Wafers and Control of Particulate Contamination Storage of Wafers and Control of Particulate Contamination with Process-Bound Wafers Summary References Index

Crystal Growth and Evaluation of Silicon for VLSI and ULSI

A suitably sized charge transfer probe of an elongated geometry can induce the formation of α-cyclodextrin nanotubular suprastructures, a rare event because of size restriction of the host. The spe...

Intramolecular Charge Transfer Probe Induced Formation of α-Cyclodextrin Nanotubular Suprastructures: A Concentration Dependent Process

In this paper, the fabrication and results of zinc oxide (ZnO) thin-film CO gas sensor are discussed The thin film of c-axis oriented ZnO is deposited on Si/SiO2 substrate at room temperature using RF magnetron sputtering technique The deposited ZnO film is annealed at different temperatures starting from 200 °C to 400 °C The crystalline structural properties are analyzed by X-ray diffraction technique and the preferred orientation of ZnO thin film (along 002 direction) is found at 400 °C Further, ZnO thin film is characterized by scanning electron microscopy and surface profilometer to analyze the surface properties and thickness The deposited ZnO film is supported by Au electrodes and Pt microheater to couple the sensing signal with outer circuitry A measurement system is also developed to measure the gas sensing properties and the results are discussed in detail

G. Eranna

Papers

Oxide Materials for Development of Integrated Gas Sensors—A Comprehensive Review

Metal Oxide Nanostructures as Gas Sensing Devices

Crystal Growth and Evaluation of Silicon for VLSI and ULSI

Intramolecular Charge Transfer Probe Induced Formation of α-Cyclodextrin Nanotubular Suprastructures: A Concentration Dependent Process

CO Sensor Using ZnO Thin Film Derived by RF Magnetron Sputtering Technique