Charge transport in semi insulating bulk 4H-Silicon carbide: Effect of metallization and wafer homogeneity

Abstract

We investigated the effect of metallization and spatial homogeneity on the charge collection of sensors prepared from high purity semi-insulating 4H-SiC bulk wafer. We used Au, Ni, Cr or graphene electrical contacts subsequently prepared on the same chip. We also tested sensors prepared from different positions on the wafer. Laser-induced transient current technique (L-TCT) and alpha spectroscopy were used to characterize the charge transport in the sensors. It was found that different sensors polarize at DC bias in the time interval of 0.2-270 s. We observed significant deviation of current transients measured on the sensor with a graphene contact from the sensors with metal contacts, which is attributed to the surface plasmon formed on the graphene-SiC interface. We demonstrated that the sensor polarization is independent of the metallization, whereas it strongly depends on the location on the wafer from which the sensor was cut. This testifies to a significant spatial inhomogeneity of the wafer. We also tested the electrical current relaxation after step-DC biasing as a simple method for the characterization of the sensor polarization. We found that the relaxation profile has extremes in the similar positions as the L-TCT profiles of collected charge. We also showed that using pulse biased L-TCT we are able to choose suitable sensors with slow polarization and to design optimal pulsing conditions including necessary depolarization times for the alpha spectroscopy measurement and L-TCT techniques to achieve long-term stable charge collection. In case of 241Am alpha spectroscopy charge collection efficiency 5-24 % was observed. We framed up model depicting all observed phenomena assuming the space charge formation induced by a blocking cathode in an n-type material and related lifetime reduction. We evaluated the space charge formed in the sensors after 10^4 s biasing, which ranged from 1.7 x 10^14 cm^-3 to 7 x 10^14 cm^-3 in respective sensors.