Effect of nitrogen implantation at the SiO2/SiC interface on the electron mobility and free carrier density in 4H-SiC metal oxide semiconductor field effect transistor channel

The electrical characteristics of n-metal oxide semiconductor field effect transistors (n-MOSFETs) fabricated on 4H-SiC with a process based on nitrogen (N) implantation in the channel region before the growth of the gate oxide are reported as a function of the N concentration at the SiO2/SiC interface. A strong correlation among the increase in the N concentration, the reduction of the interface state density near the conduction band and the improvement of the MOSFET performance was obtained. Hall-effect measurements were used to determine the electron mobility and the free carrier concentration in the MOSFET channel. Among the investigated combinations of N dose and oxidation time, the one with the higher dose and the shorter time produces MOSFETs with the higher N concentration at the SiO2/SiC interface and the best electrical characteristics. This superior performance is obtained in spite of the lowering of the bulk mobility in the channel of this sample, a negative effect probably ascribable to the incomplete recovery of the implantation damage or to the high density of interstitial nitrogen atoms present in the channel region. However, evidence of extended defects, clusters or nanoparticles was not observed by transmission electron microscopy analyses in any of the investigated SiC MOSFET devices.