Sentence:  Evidently, the annealing in NO ambient results in a <WSD>pile-up</WSD> of nitrogen at the oxide/SiC interface and this results in a reduction of interface states near the conduction band edge of 4H-SiC [16,30,31]. 
Meaning of the word marked with <WSD> tags:  An accumulation that occurs over time, especially one which is not welcome.