Studies on irradiated 4H-SiC diodes as semiconductor particle detectors

Abstract

Silicon Carbide (SiC) is a chemical compound containing silicon and carbon, which has been known in principle for more than hundred years. During the last decades, this material raised interest in various fields of science and technology. Power devices based on SiC are manufactured commercially by now and have pplications in eco-friendly electric cars, for example. Moreover, the 4H polytype (4H-SiC) is apromising candidate for particle detector technology. The reason for the interest in this material is that it exhibits advantages over conventional semiconductors like pure silicon. The most important properties with regard to detector technology is that SiC is able to withstand high radiation and temperature environments. While being operated under these conditions, a SiC devices exhibits a significantly lowerleakage current due to its wide band gap (Eg = 3.26 eV for 4H-SiC) and faster signals. In recent years, many studies have been conducted to develop detector devices based on 4H-SiC. This thesis places its emphasis on studying the performance of heavily irradiated 4H-SiC single pixel diodes with a newly designed UV-Laser setup, using the Transient Current Technique (TCT). To expose the diodes to a known equivalent fluence Φeq , the devices were irradiated in the core of the TRIGA Mark II reactor of the Atominstitut of the Technische Universität Wien. The diodes were exposed to a fluence of 5×10^14 , 1×10^15 , 5×10^15 and 1×10^16 1 MeV neutron equivalent per cm^2 . After irradiation, the devices were bonded onto a readout board and tested regarding their sensor signals as well as on the increase of their leakage current. However, even for the sensor exposed to the highest fluence only a sub-μA leakage current was obtained. In agreement with the NIEL-hypothesis, the leakage current increased with exposure to radiation. To conduct the experiments on the sensor signals, up to 1100 V reverse bias voltagewas applied onto the diodes. Afterwards, a 370 nm (3.35 eV per photon) Laser was used to excite charge carriers inside the active volume. The signals were recorded with a DRS4 oscilloscope and analyzed with respect to the following parameters: peak maximum (Vmax ), time-over-threshold (tToT ), peak area (Apeak ) and the signal-to-noise ratio (SN R). The collected data showed a decrease of all previously mentioned parameters with an increase of the fluence to which the device was exposed. It is noteworthy, that for the highest irradiated sensor no signal could be recorded. Regarding the parameters, a charge collection efficiency (CCE) was derived. The CCE exhibited 63 %, 45.8 % and 19.6 % (least to highest irradiated detector) of the maximum number of created charge carriers at the maximum applied bias voltage of 1100 V. Compensating the loss of performance due to radiation effects is possible by applying higher reverse bias voltages. However, the full depletion of the irradiated diodes was not achieved inany irradiated detector. After to the studies with the TCT-setup, a similar series of experiments were conducted at the beam line for non-clinical research at MedAustron. In this case, signals were induced with 252.7 MeV protons, depositing less energy per particle in the detector compared to the TCT-studies.In general, the data exhibited similar results. Due to uncertainties that occurred during the studies, they are considered as bare proof of concept. In conclusion, 4H-SiC sensors exposed up to fluences up to 1·10^15 neq /cm^2 are capable of showing sufficient performance to detect particles.