Defects in silicon carbide: effect on electrical and optical properties

Nowadays semiconductor detectors are widely used for ionizing radiation spectrometry and imaging in many fields such as fundamental scientific research, material science, medical applications, security systems, archeometry and many industrial applications. The highest performance required to the detectors in terms of energy and spatial resolution, room temperature operation and radiation hardness have brought to an intense international research activity on several compound semiconductors in order to overcome the limits imposed by the conventional materials (Si and Ge). The main problems arising on using compound semiconductors (GaAs, CdTe, CdZnTe, InP…) for radiation detectors are related to the crystal impurities and defects, which are responsible of charge thermal generation and trapping, and to the junction barrier height, which determines a component of the junction reverse current, which limit the signal to noise ratio and the maximum operating temperature. In the last years, silicon carbide (SiC) has been investigated as material to fabricate radiation detectors, particularly due to its intrinsic properties, which make it potentially superior to the most common semiconductor materials. This is, in particular, related to its wide band gap, resulting in low leakage current, even at high operating temperature, and to high displacement energy, making SiC detectors less susceptible to bulk radiation damage effects. Moreover, it has recently been demonstrated that a metal-SiC junction shows extremely low current densities, even at room temperature, and this feature is very favourable for the realization of low-noise X-ray semiconductor detectors. In this thesis work the results of a research activity on low-noise X-ray detectors based on SiC will be showed. The aim of this study was both to deeply understand the structural, electrical and optical properties of SiC materials available for the device realisation and, at the same time, to investigate the effects of irradiation on SiC properties. A fundamental understanding of defect creation, accumulation, and recovery in irradiated SiC is in fact needed to advance the device fabrication and evaluate their performance in high-radiation environments. The first step of this work has been a wide-ranging characterization of as-grown SiC substrates and epilayers. Later on, the detectors have been irradiated with electrons and neutrons in order to preliminarily test their radiation hardness and the resulting damage has mainly been studied by Photoluminescence (PL) and the Deep Level Transient Spectroscopies (DLTS) and by monitoring the minority carrier diffusion length of the samples. To this purpose two different methods for the determination of the minority carriers diffusion length have been developed, the former based on a spectral response technique, the latter based on OBIC (Optical Beam Induced Current) mapping.