Passivation of 4H–SiC Schottky barrier diodes using aluminum based dielectrics

Passivation is a key technological step for achieving high voltage devices and needs to be thoroughly addressed. The passivation materials commonly used for SiC devices are mainly adapted from the silicon (Si) technology. However, these may not necessarily be suitable for SiC devices and may compromise their performance. In this study a variety of dielectrics, namely PECVD silicon dioxide (SiO2), sputter deposited aluminum nitride (AlNx), hydrogenated aluminum nitride (AlNy:H) and aluminum oxide (AlOz) with wide ranging dielectric constants (k) were deposited on unterminated 4H–SiC Schottky barrier diodes (SBDs) as passivation. Some of the devices in this study have a thin interfacial layer of SiO2 thermally grown on the surface while for the other devices this oxide was stripped prior to passivation deposition. On all these devices, diode characteristics in terms of reverse bias leakage current density (Jr), ideality factor (η), Schottky barrier height (ϕb), etc. have been studied. The effects of increasing k on the leakage currents were analyzed since a larger k has a direct impact on reducing the electric field enhancement at the Schottky contact corner or periphery. Our results indicate that for diodes with a passivation layer consisting of thermal oxide interfacial layer and a deposited layer of AlNy:H there is a remarkable reduction in the leakage currents by more than two orders of magnitude and an increase in ϕb by as much as 0.2 eV when compared to devices without the AlNy:H. This is a direct result of lowering in surface states in excess of 30% as a result of AlNy:H passivation. In general the Al based passivations perform better than the conventional SiO2 passivation providing at least twice the amount of leakage current reduction due to lower edge related tunneling leakage currents. Other advantages of Al based passivations include much better heat dissipation due to high thermal conductivity which suppresses surface traps. On the other hand, diodes with a passivation layer consisting of just the deposited Al based passivation and without the interfacial thermal oxide layer showed degraded performance, attributed to the poor interface, increased leakage through the dielectric as result of reduced band offset and sputter-induced damage on the surface.