The silicon-nitride layer on SI surfaces is one of the most important issues not only in sikicon devices technology but also in surface science and has been studied using various techniques. In the case of nitridation on a Si(111) surface, the ordered atomic structure of the nitrogenized surface has been characterized by the `8x8' or the `3/4x3/4' (quadruplet) depending on nitridation temperature. According to the experiment using low energy electrn diffraction (LEED), the basic unit cell of the `8x8' surface shows a 8/11x8/11 triangular lattice with respect to the Si(111)-1x1 lattice, and that of the `3/4x3/4' surface shows a 3/4x3/4 triangular lattice with rotation of +-5 and +-10 deg with respect to the Si(111)-1x1 lattice. In the case of 8x8 surface, atomically flat terraces with the 8/3x8/3 dots can be observed by STM measurement. However, there has been no consistent structural models for 8/3x8/3 (STM) image and 8x8 (LEED) superstructure.
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