| Abstract |
The chemical sensors are important analytical instruments that are used for the quantitative and qualitative analysis of a variety of analytes. Their value is shown by their continuously increasing analytical market share, as well as in clinical, environmental and industrial analysis. They are the basis for the development of a large range of chemical analysis systems of significant commercial and scientific value.
The most important parameter of a sensor is the selective and chemically active surface. This surface is responsible for the selective and recognition of the analyte and subsequently determines the analytical characteristics of the sensor. Such analytical instruments have the ability to determine and monitor the analyte with great precision in a range of concentrations, usually from 10-6M to roughly 1M.
The aim of present thesis is the development and study of bew chemical sensors based on the semiconductors of group III-Nitrides, and in particular on Gallium Nitride (GaN). THe surface of the (0001)-oriented Gallium Nitride (GaN) crystal can act as the sensing element of the sensor creating a new generation of solid state chemical sensors. Alternatively, the Heterojuction AlGaN/GaN can be used for the development of a new Ion Selective Field Effect Transistor (ISFET).
The last decades, Silicon (Si) is the most widespread semiconductor element. However, the development of III-Nitrides extended the applications in regions where the usual semiconductors, as the Silicon (Si), the Germanium (Ge) and the Gallium Arsenide (GaAs) could never be applied. Wide bandgap semiconductors such as the group of III-Nitrides (GaN, AlN and their alloys are not only a subject of research in the section of microelectronics, but they have begun to occupy also a percentage og international market of semiconductor elements. The III-Nitrides are of intense research lately, mainly due to the improvements in their fabrication technology. Their unique physical and chemical properties allow for their application in harsh environments. These materials are being used for the development of high power and high frequency electronic devices and visible - UV optoelectronic devices. In particular, they are the only semiconductors that can emit light efficiently at small wavelenghts (blue-ultravioloet). Moreover, this particular group of semiconductors presents higher thermal conductivity, chemical inertness, mechanical stability and resistance in chemical erosion.
In the first chapter, the (0001)-oriented Gallium Nitride (GaN) crystal, which was grown by Molecular Beam Epitaxy (MBE), was used as the sensor element for the development of a new solid state potentiometric sensor. It is shown that the response of the sensor is based on the development of potential difference in the interface between the crystal and the solution, caused by the selective interaction of Gallium atoms of crystal surface with anions of the solution. It was also observed that the selectivity and the sensitivity of the sensor depend mainly on the quality of the crystal's surface, which determines the number of the fixed sites that interact with the analyte. In addition, the analytical characteristics of GaN based sensors are shown to depend from the type of the chemical bond that exists between the analyte and the Gallium atoms.
In the second chapter, the heterojuction of AlGaN/GaN has been used for the development of a new Ion Selective Field Effect Transistor (ISFET). These chemical sensors are currently, based on the mature technology of silicon integrated circuits (ICs), which allows their automatization and minimization. A strong polarization exhibits in the layers of the AlGaN/GaN heterojuction, resulting in the spontaneous formation of a two dimensional electron gas (2DEG) at the interface between GaN and AlGaN. This conductive channel -2DEG- is very sensitive to changes in ambient environments to which the GaN surface (gate area) is exposed. Improvement in epitaxial growth methods allow for the design of devices with a 2DEG located closely to the active surface (100-150 A). This characteristic is shown to be a very significant development in the ISFETs area of research.
The microsensor developed, has shown sensitivity to pH and to anions, which is attributed to the surface of (0001) Gallium Nitride crystal. Moreover, the selectivity of the microsensor can be modiefied using polymeric active membranes which are deposited onto the sensing element. In the latter case, the use of AlGaN/GaN Heterojuction FIeld Effect Transistors (HFETs), as signal transducers has been studied using cation selective polymeric membranes.
The results presented in this work show the advantage of using (0001)-oriented Gallium Nitride either in its crystal structure or in Heterojuction with AlGaN, for the development of a new generation of solid state chemical sensors.
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Ανάπτυξη και μελέτη διατάξεων HFET του ημιαγωγού GaN για εφαρμογές σε χημικούς αισθητήρες
