A classification of chemically sensitive semiconductor devices

A general scheme is presented for classifying chemically sensitive semi-conductor devices (CSSDs). CSSDs reported in the literature up to now, as well as related physicochemical phenomena, are briefly discussed and shown to fit in the scheme

Wide band gap materials and devices for NOx, H2 and O2 gas sensing applications

Im Rahmen dieser Arbeit sind Feldeffektgassensoren (Schottky Dioden, MOS Kapazitäten, und MOSFET Transistoren) auf der Basis von Halbleitern mit großer Bandlücke (Siliziumkarbid (SiC) und Gallium Nitrid (GaN), sowie resistive Gassensoren, die auf aktiven Indiumoxid-Schichten (In2O3) basieren, für die Detektion von reduzierenden Gasen (H2, D2) und oxidierenden Gasen (NOx, O2), entwickelt worden. Die Entwicklung der Sensoren ist am Institut für Mikro- und Nanoelektronik der Technischen Universität Ilmenau in Zusammenarbeit mit General Electric (GE) Global Research (USA) und der Umwelt- und Sensortechnik GmbH (Geschwenda) durchgeführt worden. Kapitel 1: dient als eine Einführung in das mit dieser Arbeit verbundene wissenschaftliche Feld. Die theoretischen Grundlagen der Festkörper-Gassensoren werden dargestellt. Zusätzlich werden in diesem Kapitel die relevanten Eigenschaften der Materialien mit großer Bandlücke (SiC und GaN) präsentiert. Kapitel 2: Pt/GaN Schottky Dioden mit verschiedener Dicke des katalytischen Metalls werden als Wasserstoffgasdetektoren vorgestellt. Die Fläche sowie die Dicke von Pt-gates wurden zwischen 250 × 250 µm2 und 1000 × 1000 µm2, 8 und 40 nm, systematisch variiert. Die Sensorantwort (Sensorsreaktion) auf 1 vol.% Wasserstoff in synthetischer Luft wurde in Abhängigkeit von der aktiven Fläche, der Pt-Dicke, und der Betriebstemperatur untersucht. Durch Anheben der Betriebstemperatur auf ca. 350°C und durch Reduzierung der Dicke des Pt auf 8 nm beobachteten wir eine beträchtliche Erhöhung der Empfindlichkeit sowie eine Verkürzung der Ansprech- und Erholzeiten. Untersuchungen am Elektronenmikroskop zeigten, dass das dünnere Platin eine höhere Korngrenzendichte aufwies. Die Erhöhung der Empfindlichkeit gemeinsam mit der Reduzierung der Dicke des Pt deuten auf die Dissoziierung von molekularem Wasserstoff an der Oberfläche, die Diffusion atomaren Wasserstoffs entlang der Korngrenzen des Platins und die Adsorption von Wasserstoff an der Pt/GaN Grenzfläche als ein möglicher Mechanismus der Detektion von Wasserstoff durch Schottky Dioden hin. Die Reaktion auf D2, NOx, and O2 von Metall-Oxid-Halbleiter (MOS) Strukturen mit Rhodium Schottky-Kontakten mit einer Dicke von 30 nm in Abhängigkeit von der Betriebstemperatur und der Gaspartialdrücke wurde in Kapitel 3 untersucht. Die Reaktion dieses Gates wurde als Verschiebung entlang der Spannungsachse in der Kapazität-Spannungs Kurve (C-V) nachgewiesen. Positive und negative Flachband-Verschiebungen jeweils bis zu 1 V wurden für oxidierende und reduzierende Gase beobachtet. Abhängig vom gewählten Typ des Isolators wurden Unterschiede in den Empfindlichkeiten beobachtet. In Kapitel 4: SiC-basierten FETs mit verschiedenen Materialien für das Gate (Gemisch aus Metalloxiden: Indiumoxide und Zinnoxid (InxSnyOz), Indiumoxid und Vanadiumoxid (InxVyOz) sowie ein Gemisch aus Metalloxiden mit Zugabe einer entsprechenden Menge Metallzusätzen) wurden als NOx, O2, und D2 Gasdetektoren untersucht. Die Reaktion auf diese Gase wurde in Abhängigkeit von der Betriebstemperatur und der Gaspartialdrücke untersucht. Die Zusammensetzung der aktiven Metalloxid-Schicht und die Mikrostruktur der sensitiven Gateelektrode sind die entscheidenden Parameter mit Einfluss auf den Messmechanismus und somit die entscheidenden Leistungsparameter des Sensors: Empfindlichkeit, Selektivität und Reaktionszeit. Durch die Optimierung der Temperatur und des richtigen Materials des Katalysators können Sensoren mit sehr hoher Empfindlichkeit gegenüber relevanten Gasen realisiert werden. Wird auch der Katalysator sorgfältig ausgewählt, können diese Erkenntnisse für eine Erhöhung der Selektivität des Sensors genutzt werden. In Kapitel 5: Polykristalle von 200 nm Dicke und 10 nm nanostrukturierten Dünnschichten aus In2O3, die durch MOCVD (metallorganische Gasphasenabscheidung) gewachsen sind, wurden untersucht, um Informationen über ihre Eigenschaften hinsichtlich der Detektion von NOx- and O2-Gasen zu erhalten. Die Reaktion auf diese Gase wurde in Abhängigkeit von der Betriebstemperatur und der Gaspartialdrücke untersucht. Die Experimente in Anwesenheit verschiedener Partialdrücke des NOx haben gezeigt, das beide Dünnschichten in der Lage sind, Stickoxide zu detektieren. Es wurde festgestellt, dass besonders die nanostrukturierte In2O3-Dünnschicht stärker auf NOx reagiert. Dieser Effekt wird durch das höhe Oberflächen-zu-Volumenverhältnis infolge der niedrigen Korngröße verbessert, so dass der relative interaktive Oberflächenbereich größer und die Dichte der Ladungsträger höher ist. Wir haben ermittelt, dass die Reduzierung der Korngröße des messenden Materials auf unter 10 nm erhebliche Auswirkung auf die Empfindlichkeit des Sensors hat. Die hinsichtlich der Empfindlichkeit und Reaktion optimalen Temperaturen des nanostrukturierten In2O3 für den Nachweis von NOx treten im Bereich von 100-175°C auf. In diesem Temperaturbereich ist die Reaktion auf O2 sehr schwach, was darauf hinweist, das der Sensor für die selektive Erkennung von NOx bei niedrigen Temperaturen sehr gut geeignet ist. Zudem wurde festgestellt, dass die nanostrukturierte In2O3-Dünnschicht für den Einsatz in der Erkennung niedriger Partialdrücke die optimale ist. Kapitel 6 enthält Schlussfolgerungen aus den gegenwärtigen Arbeiten. In diesem Kapitel vergleichen wir alle untersuchten Gassensoren in Bezug auf deren Empfindlichkeit, Selektivität und Reaktionszeit und stellen diese anschließend den entsprechenden Ergebnissen anderer, in der wissenschaftlichen Literatur zu findenden Autoren gegenüber.In this thesis, field effect gas sensors (Schottky diodes, MOS capacitors, and MOSFET transistors) based on wide band gap semiconductors like silicon carbide (SiC) and gallium nitride (GaN), as well as resistive gas sensors based on indium oxide (In2O3), have been developed for the detection of reducing gases (H2, D2) and oxidising gases (NOx, O2). The development of the sensors has been performed at the Institute for Micro- and Nanoelectronic, Technical University Ilmenau in co-operation with (GE) General Electric Global Research (USA) and Umwelt-Sensor-Technik GmbH (Geschwenda). Chapter 1: serves as an introduction into the scientific fields related to this work. The theoretical fundamentals of solid-state gas sensors are provided and the relevant properties of wide band gap materials (SiC and GaN) are summarized. In chapter 2: The performance of Pt/GaN Schottky diodes with different thickness of the catalytic metal were investigated as hydrogen gas detectors. The area as well as the thickness of the Pt were varied between 250 × 250 µm2 and 1000 × 1000 µm2, 8 and 40 nm, respectively. The response to hydrogen gas was investigated in dependence on the active area, the Pt thickness and the operating temperature for 1 vol.% hydrogen in synthetic air. We observed a significant increase of the sensitivity and a decrease of the response and recovery times by increasing the temperature of operation to about 350°C and by decreasing the Pt thickness down to 8 nm. Electron microscopy of the microstructure showed that the thinner platinum had a higher grain boundary density. The increase in sensitivity with decreasing Pt thickness points to the dissociation of molecular hydrogen on the surface, the diffusion of atomic hydrogen along the platinum grain boundaries and the adsorption of hydrogen at the Pt/GaN interface as a possible mechanism of sensing hydrogen by Schottky diodes. The response to deuterium D2, NOx, and O2 of metal-oxide-semiconductor (MOS) and metal-metal oxide-oxide-semiconductor (MMOOS) structures with rhodium (Rh) gate were investigated in dependence on the operating temperature and gas partial pressures was investigated in chapter 3. The response of the sensor was measured as a shift in the capacitance-voltge (C-V) curve along the voltage axis. Positive and negative flat-band voltage shifts up to 1 V were observed for oxidizing and reducing gases, respectively. Depending on the type of insulator that is chosen, differences in the sensitivity of the sensor were observed. In chapter 4: The performance of SiC-based field effect transistors (FETs) with different gate materials (mixture of metal oxides: indium oxide and tin oxide (InxSnyOz), indium oxide and vanadium oxide (InxVyOz), as well as mixtures of metal oxides with metal additives) were investigated as NOx, O2, and D2 gas detectors. The response to these gases was investigated in dependence on the operating temperature and gas partial pressures. The composition and microstructure of the sensing gate electrode are the key parameters that influence the sensing mechanism, and hence key performance parameters: sensitivity, selectivity, and response time. By choosing the appropriate temperature and catalyst material (gate material), devices that are significantly sensitive to certain gases may be realized. In addition, the temperature of maximum response varies dependent on the gas species being measured. This information, along with a careful choice of catalyst (gate material) can be used to enhance device selectivity. In chapter 5: Polycrystalline and nano-structured In2O3 thin films were investigated with the aim to obtain information about their NOx and O2 gas sensing properties. The response to these gases was investigated in dependence on the operating temperature and gas partial pressures. The analysis in the presence of different partial pressures of NOx has shown that both thin films are able to detect nitrogen oxide, but their responses exhibit different characteristics. In particular, nano-structured In2O3 thin films were found to have the higher response to NOx. This is most probably due to the enlarged overall active surface area of the sensing layer as a consequence of the small grain size (higher surface to volume ratio) so that the relative interactive surface area is larger, and the density of charged carriers per volume is higher. We have found that reducing the grain size of the sensing material to the ~10 nm regime can have a substantial effect on performance. The optimum detection temperatures of the nano-structured In2O3 occur in the range of 100-175°C for NOx considering the sensitivity as well as the response time. In this range of temperatures the response to O2 is very low indicating that the sensor is very suitable for selective detection of NOx at low temperatures In addition, nano-structured In2O3 thin films were found to be more suitable to be used in the field of application for detecting low partial pressures. Chapter 6: offers conclusions of the current work. In this chapter we compare also all studied gas sensors according to their sensitivity, selectivity, and response time and then we compare them with the related works by other authors available in the scientific literature

Feature Papers in Electronic Materials Section

This book entitled "Feature Papers in Electronic Materials Section" is a collection of selected papers recently published on the journal Materials, focusing on the latest advances in electronic materials and devices in different fields (e.g., power- and high-frequency electronics, optoelectronic devices, detectors, etc.). In the first part of the book, many articles are dedicated to wide band gap semiconductors (e.g., SiC, GaN, Ga2O3, diamond), focusing on the current relevant materials and devices technology issues. The second part of the book is a miscellaneous of other electronics materials for various applications, including two-dimensional materials for optoelectronic and high-frequency devices. Finally, some recent advances in materials and flexible sensors for bioelectronics and medical applications are presented at the end of the book

Graphene Schottky diodes: an experimental review of the rectifying graphene/semiconductor heterojunction

In the past decade graphene has been one of the most studied material for several unique and excellent properties. Due to its two dimensional nature, physical and chemical properties and ease of manipulation, graphene offers the possibility of integration with the exiting semiconductor technology for next-generation electronic and sensing devices. In this context, the understanding of the graphene/semiconductor interface is of great importance since it can constitute a versatile standalone device as well as the building-block of more advanced electronic systems. Since graphene was brought to the attention of the scientific community in 2004, the device research has been focused on the more complex graphene transistors, while the graphene/semiconductor junction, despite its importance, has started to be the subject of systematic investigation only recently. As a result, a thorough understanding of the physics and the potentialities of this device is still missing. The studies of the past few years have demonstrated that graphene can form junctions with 3D or 2D semiconducting materials which have rectifying characteristics and behave as excellent Schottky diodes. The main novelty of these devices is the tunable Schottky barrier height, a feature which makes the graphene/semiconductor junction a great platform for the study of interface transport mechanisms as well as for applications in photo-detection, high-speed communications, solar cells, chemical and biological sensing, etc. In this paper, we review the state-of-the art of the research on graphene/semiconductor junctions, the attempts towards a modeling and the most promising applications.Comment: 85 pages. Review articl

Silicon Carbide Technology

Silicon carbide based semiconductor electronic devices and circuits are presently being developed for use in high-temperature, high-power, and high-radiation conditions under which conventional semiconductors cannot adequately perform. Silicon carbide's ability to function under such extreme conditions is expected to enable significant improvements to a far-ranging variety of applications and systems. These range from greatly improved high-voltage switching for energy savings in public electric power distribution and electric motor drives to more powerful microwave electronics for radar and communications to sensors and controls for cleaner-burning more fuel-efficient jet aircraft and automobile engines. In the particular area of power devices, theoretical appraisals have indicated that SiC power MOSFET's and diode rectifiers would operate over higher voltage and temperature ranges, have superior switching characteristics, and yet have die sizes nearly 20 times smaller than correspondingly rated silicon-based devices [8]. However, these tremendous theoretical advantages have yet to be widely realized in commercially available SiC devices, primarily owing to the fact that SiC's relatively immature crystal growth and device fabrication technologies are not yet sufficiently developed to the degree required for reliable incorporation into most electronic systems. This chapter briefly surveys the SiC semiconductor electronics technology. In particular, the differences (both good and bad) between SiC electronics technology and the well-known silicon VLSI technology are highlighted. Projected performance benefits of SiC electronics are highlighted for several large-scale applications. Key crystal growth and device-fabrication issues that presently limit the performance and capability of high-temperature and high-power SiC electronics are identified

Detection mechanism in highly sensitive ZnO nanowires network gas sensors

Metal-oxide nanowires are showing a great interest in the domain of gas sensing due to their large response even at a low temperature, enabling low-power gas sensors. However their response is still not fully understood, and mainly restricted to the linear response regime, which limits the design of appropriate sensors for specific applications. Here we analyse the non-linear response of a sensor based on ZnO nanowires network, both as a function of the device geometry and as a response to oxygen exposure. Using an appropriate model, we disentangle the contribution of the nanowire resistance and of the junctions between nanowires in the network. The applied model shows a very good consistency with the experimental data, allowing us to demonstrate that the response to oxygen at room temperature is dominated by the barrier potential at low bias voltage, and that the nanowire resistance starts to play a role at higher bias voltage. This analysis allows us to find the appropriate device geometry and working point in order to optimize the sensitivity. Such analysis is important for providing design rules, not only for sensing devices, but also for applications in electronics and opto-electronics using nanostructures networks with different materials and geometries

Miniaturized Silicon Photodetectors

Silicon (Si) technologies provide an excellent platform for the design of microsystems where photonic and microelectronic functionalities are monolithically integrated on the same substrate. In recent years, a variety of passive and active Si photonic devices have been developed, and among them, photodetectors have attracted particular interest from the scientific community. Si photodiodes are typically designed to operate at visible wavelengths, but, unfortunately, their employment in the infrared (IR) range is limited due to the neglectable Si absorption over 1100 nm, even though the use of germanium (Ge) grown on Si has historically allowed operations to be extended up to 1550 nm. In recent years, significant progress has been achieved both by improving the performance of Si-based photodetectors in the visible range and by extending their operation to infrared wavelengths. Near-infrared (NIR) SiGe photodetectors have been demonstrated to have a “zero change” CMOS process flow, while the investigation of new effects and structures has shown that an all-Si approach could be a viable option to construct devices comparable with Ge technology. In addition, the capability to integrate new emerging 2D and 3D materials with Si, together with the capability of manufacturing devices at the nanometric scale, has led to the development of new device families with unexpected performance. Accordingly, this Special Issue of Micromachines seeks to showcase research papers, short communications, and review articles that show the most recent advances in the field of silicon photodetectors and their respective applications