Wide-bandgap detectors for low-flux radiations and laser-generated plasma diagnostics

This thesis is the result of the three years of research carried out mainly at Plasma Physics laboratories at the University of Messina, but also in National and European laboratories. The main research topic concerned the study and development of semiconductor devices, and in particular Silicon Carbide based devices. Silicon Carbide, more commonly called SiC, is a material that thanks to its excellent mechanical properties and high hardness is widely used in a wide class of applications, such as ballistic protection, in the automotive industry, in the field of brake systems, in the production of particulate filters, in the nuclear physics branch such as coating of nuclear fuel elements, in medicine for imaging applications and in many other fields. Also in the field of Microelectromechanical systems (MEMS) there are very promising results in the production of pressure sensors, accelerometers, resonant structures, motors, wear resistant devices, chemical sensor, gas sensor devices, microhotplates, optical devices and light emitted diodes. In recent years, however, research has been intensied in the field of power electronic devices. Driven by the progress made in the crystal grow technique, SiC-based technology is what seems to show greater prospects for large-scale production and trade. Currently, SiC is used to produce power inverters and is already produced by several companies world leader in semiconductor solutions such as, Infineon Technologies AG, SEMIKRON or STMicroelectronics, that has been producing this type of SiC-based devices since 2008. These diodes thanks to the physical and chemical properties offered by this materials, have excellent qualities such as optimum switching performance and need a lower thickness to support the same breakdown voltage compared to conventional diodes. Closely related to this area of development there is the innovative research, carried out even in our laboratories, which is devoted to demonstrate the possible usage of this device also as radiations detectors. In this contest the benets of SiC can be summarized in: high charge collection speed, high resistance to radiation-induced damage, high Thermal conductivity and high saturation current speed. Moreover, considering the wide band gap of the material, it is possible to use the detectors at room temperature, since even under such conditions there is a very low dark current density that ranges from 10^{-10} A/cm^2 at a bias voltage of 200V up to 10^{-8} A/cm^2 for a voltage of 700 V. Another consequence of the high band gap is the fact that these devices are blind to visible radiation and to soft UV, allowing the possibility of increasing sensitivity to the detection of heavier ions and protons even in a strongly compromised electromagnetic environment. But one of the most attractive properties of this material is the high resistance to radiation-induced damage due to higher bonding energy and greater displacement energy than traditional semiconductors. These features make the SiC detectors one of the most valuable silicon alternatives in all those experimental situations characterized by a harsh environment, i.e. high doses, high frequency and high radiation-induced damage. There are already promising results in literature on the detection of neutrons, X- and gamma rays and charged particles, but the most interesting application of SiCs concerns the monitoring of radiations emitted by laser generated plasma. Plasma offers innovative applications in many fields such as medicine, industry, but especially in the field of nuclear physics and so. Among the unique features that plasmas offer there is the very high brightness of the ion, electron or photon beam that can be sent in short-duration pulses and the extreme compactness of the source. It should be noted that in this field the energy spectrum of the radiations emitted by the plasma is extremely broad as well as the type of particles emitted and therefore extremely versatile devices are required. SiC detectors are very useful in these particular physical conditions in fact technological advances have made it possible to produce diodes with different geometries and characteristics, giving the possibility to choose the best device based on the experimental condition investigated. On the one hand there are SiC devices with a deep sensitive region, suitable for very energetic ions or X-rays radiation, on the other hand there are also detectors with very thin metals and small regions sensitive to less penetrating radiations such as electrons, light ions and UV radiation. Moreover by combining the results of these detectors with devices such as Faraday cup, Ion Energy Analyzer or other diagnostic techniques, it is possible to give a complete characterization of plasma in terms of angular, velocity and energy distribution of radiation emitted and of temperature and plasma density. It should be noted, however, that SiC is not the only wide bandgap material that can be used for such applications. The diamond, for example, or Gallium Arsenide, are the main competitors of Silicon Carbide. However, it should be noted that for these materials, development processes are not yet optimized and are very costly. The thesis is therefore entirely focused on the comparison of detectors based on SiC and other semiconductor materials, mainly Silicon and Diamond. This comparison will be carried out not only at low fluence where a spectroscopic electronic chain can be used, but above all in the high fluxes regime where a time of flight (TOF) technique is needed. In the realization of this thesis, the wide series of experimental measurements performed in different national and European laboratories has been enriched with an intense bibliography work to highlight the state of the art on such devices.