Scanning tunneling microscope studies on twinned atomic structures of 19×19 surface reconstruction in the Ni/Si(111) system

It has been known that the surface reconstruction of 19×19 (hereafter referred to as 19) is induced on to the Si(111) surface by introducing a small amount of Ni adsorbates. The 19 reconstruction has been extensively studied by Auger electron spectroscopy (AES), reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). In the present paper, we shall report the first successful preparation of 19 by melting small particles of Ni on the Si(111) surface in the STM chamber. The STM observations of the resultant 19 show that there are two types of domains in 19, and they are found to be twin-oriented. A simple crystallographic argument shows that a pair of twin-oriented unit cells are possible in 19. We propose an atomic model of the 19 reconstruction on the basis of the present STM observation together with previous studies. esent STM observation also shows that the phases of 7×7, 1×1-RC and 19 appear in this order as the Ni content increases. The 1×1-RC phase is nucleated along a lower step edge of the Si(111) sample heterogeneously, while the 19 phase is nucleated homogeneously on the terrace.