Radiation Hardness of 4H-SiC Devices and Circuits

University dissertation from KTH Royal Institute of Technology

Abstract: Advances in space and nuclear technologies are limited by the capabilities of the conventional silicon (Si) electronics. Hence, there is a need to explore materials beyond Si with enhanced properties to operate in extreme environments. In this regards, silicon carbide (4H-SiC), a wide bandgap semiconductor, provides suitable solutions. In this thesis, radiation effects of 4H-SiC bipolar devices, circuits and dielectrics for SiC are investigated under various radiation types. We have demonstrated for the first time the radiation hardness of 4H-SiC logic circuits exposed to extremely high doses (332 Mrad) of gamma radiation and protons. Comparisons with previously available literature show that our 4H-SiC bipolar junction transistor (BJT) is 2 orders of magnitude more tolerant under gamma radiation to existing Si-technology. 4H-SiC devices and circuits irradiated with 3 MeV protons show about one order of magnitude higher tolerance in comparison to Si.Numerical simulations of the device showed that the ionization is most influential in the degradation process by introducing interface states and oxide charges that lower the current gain. Due to the gain reduction of the BJT, the voltage reference of the logic circuit has been affected and this, in turn, degrades the voltage transfer characteristics of the OR-NOR gates.One of the key advantages of 4H-SiC over other wide bandgap materials is the possibility to thermally grow silicon oxide (SiO2) and process device in line with advanced silicon technology. However, there are still questions about the reliability of SiC/SiO2 interface under high power, high temperature and radiation rich environments. In this regard, aluminium oxide (Al2O3), a chemically and thermally stable dielectric, has been investigated. It has been shown that the surface cleaning treatment prior to deposition of a dielectric layer together with the post dielectric annealing has a crucial effect on interface and oxide quality. We have demonstrated a new method to evaluate the interface between dielectric/4H-SiC utilizing an optical free carrier absorption technique to quantitative measure the charge carrier trapping dynamics. The radiation hardness of Al2O3/4H-SiC is demonstrated and the data suggests that Al2O3 is better choice of dielectric for devices in radiation rich applications.