The Effect of Nitridation on SiC MOS Oxides as Evaluated by Charge Pumping

We have analyzed the effect of post-oxidation nitride anneals (usually with either NO or N2O gases) on SiC MOSFETs. Two 4H:SiC wafers were identically prepared except that one wafer had a nitridation anneal after the gate oxide was formed, while the other was tested as-oxidized. We compared the two processes by making measurements on lateral MOSFETs and MOS capacitors using ID-VGS, C-V, and charge pumping. There was no change in either flatband voltage or interface trap density near the valence band, suggesting that the net fixed charge remained constant (within a few 1011cm-2). However, there was a large shift in the threshold voltage which, when combined with the C-V results, indicates a strong reduction of interface traps near the conduction band of roughly 6.0x1012cm-2 by using the nitridation process. The charge pumping measurements also showed a strong reduction of interface traps. Charge pumping measured a trapping density of 2.5x1012cm-2 for the as-oxidized samples and 5.3x1011cm-2 for the nitrided samples. The frequency-dependence of the charge pumping signal also indicates a spatial distribution of traps, with volumetric trap densities of roughly 1.3x1019cm-3 over 25Å on as-oxidized and 3.8x1018cm-3 over 19Å for nitrided.