Effects of Surface Hydrogenation on Initial Reaction Processes of Transition-Metal Adatoms on Silicon Surfaces
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Effects of Surface Hydrogenation on Initial Reaction Processes of Transition-Metal Adatoms on Silicon Surfaces Shin’ichi Higai and Takahisa Ohno National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ABSTRACT We studied the effects of the surface hydrogenation on the adsorption, penetration, and silicidation, i.e., the initial reaction processes of the transition-metal Ni and Ti adatoms on the Si surfaces by the first-principles theoretical calculations. We found for both Ni and Ti that the surface hydrogenation changes the most stable surface site and reduces the adsorption energy. In addition, it blocks the penetration, and thus prevents the silicidation. Furthermore, we newly propose its interesting effects from our results, i.e., impurity metal atoms existing in the Si subsurface are extracted onto the surface by the surface hydrogenation. Thereby, highly pure and atomically flat Si surfaces are expected to be obtained. INTRODUCTION Much recent attention is directed to the hydrogenation of Si surfaces, because of its potential to modify the surface reactions of various impinging atoms and molecules, and moreover, to control the growth of various thin films on them. On Si surfaces, almost all kinds of metal react with the surface to form metal-Si compounds, i.e., silicides. Among numerous kinds of silicides, the 3d transition-metal (TM) Ni- and Ti-silicides are especially important, because they are excellent contact materials for Si devices. On growth of the silicide films on Si surfaces, the control of the interfaces between silicides and Si with a monolayer thickness is an important problem, because it affects contact properties such as the Schottky barrier height. In recent experiments, Yoshimura et al. [1] showed for Ni that the surface hydrogenation considerably changes its initial growth mode on the Si(001) surface, using scanning tunneling microscopy (STM). On the bare Si surface, adsorbed Ni atoms immediately react with the surface and induce dimer vacancy (DV) defects at room temperature. On the H-terminated (H/) Si surface, on the other hand, they do not react with the surface, instead they clusterize on it. In the present study, we theoretically investigate the adsorption, penetration, and silicidation, i.e., the initial reaction processes of the Ni and Ti adatoms on both the bare Si(001) and H/Si(001)-(2×1) surfaces based on first-principles calculations, in order to clarify the effects of the surface hydrogenation on them.
METHOD We briefly explain the method of our study. The first-principles calculations that we perform are based on the density functional formalism [2, 3] with the generalized gradient approximation [4]. For the atomic potentials, we adopt two types of pseudopotentials: the norm-conserving type [5] for the Si atom and the ultra-soft type [6] for the Ni, Ti, and H atoms. The partial core P11.33.1
correction [7] is employed for Ni and Ti. We use the plane wave basis set up to the cut-off energy of 20.25 Ry. The bare Si(001) and H/Si(001)-(2×1) surfa
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