Interaction of nitrogen with Si(111)-7 × 7 surfaces at elevated temperatures

The interaction of nitrogen with Si(111)-7 × 7 surfaces at elevated temperatures ranging from 600 to 900°C was investigated by using a scanning tunneling microscope (STM). Silicon nitride islands in a nanometer scale were formed upon nitrogen exposure at temperatures above 700°C. In the initial stage of nitridation, those islands were formed preferentially at the step edges and then additional ones were formed on the Si terrace with further nitridation. The growth of the silicon nitrides showed a strong preference along the 〈1̄1̄2〉 directions, resulting in triangular islands, and such a trend became more prominent at higher temperatures. Silicon nitride islands showed an 8 × 8 superstructure that appeared as an ordered arrangement of bright protrusions in a three-fold symmetry. By STM measurements of the surface morphology, we show the influences of the nitridation and post-annealing temperatures on the island size distribution, island density, and total surface coverage. Based on the experimental results, two possible mechanisms of thermal nitridation are suggested.