Characterization of dielectric layers for passivation of 4H-SiC devices

Abstract: Silicon carbide rectifiers and MESFET switches are commercially available since 2001 and 2005 respectively. Moreover, three inch SiC wafers can be purchased nowadays without critical defects for the device performance, four inch wafers are available and the next step of technology is set to be the six inch substrate wafers. Despite this tremendous development in SiC technology the reliability issues, like bipolar device degradation, passivation, or low MOSFET channel mobility still remain to be solved. This thesis focuses on SiC surface passivation and junction termination, a topic which is very important for the utilisation of the full potential of this semiconductor. Five dielectrics with high dielectric constants, Al2O3, AlN, AlON, HfO2 and TiO2 have been investigated. The layers were deposited directly on SiC, or on the thermally oxidized SiC surfaces with several different techniques. The structural and electrical properties of the dielectrics were measured and the best insulating layers were then deposited on fully processed and well characterised 1.2 kV 4H SiC PiN diodes. For the best Al2O3 layers, the leakage current was reduced to half its value and the breakdown voltage was extended by 0.7 kV, reaching 1.6 kV, compared to non passivated devices. Furthermore, AlON deposited on 4H SiC at room temperature provided interface quality comparable to that obtained with the thermally grown SiO2/SiC system. As important as the proper choice of dielectric material is a proper surface preparation prior to deposition of the insulator. In the thesis two surface treatments were tested, a standard HF termination used in silicon technology and an exposure to UV light from a mercury or deuterium lamp. The second technique is highly interesting since a substantial improvement was observed when UV light was used prior to the dielectric deposition. Moreover, UV light stabilized the surface and reduced the leakage current by a factor of 100 for SiC devices after 10 Mrad γ ray exposition. The experiments show also that the measured leakage currents of the order of pA are dominated by surface leakage.