Two-Stage Rotation Mechanism for Group-V Precursor Dissociation on Si(001) Jian-Tao Wang, 1 Changfeng Chen, 2 E. G. Wang, 1 Ding-Sheng Wang, 1 H. Mizuseki, 3 and Y. Kawazoe 3 1 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China 2 Department of Physics, University of Nevada, Las Vegas, Nevada 89154, USA 3 Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan (Received 13 March 2006; published 26 July 2006) We report ab initio identification of initial dissociation pathways for Sb 4 and Bi 4 tetramer precursors on Si(001). We reveal a two-stage double piecewise rotation mechanism for the tetramer to ad-dimer conversion involving two distinct pathways: one along the surface dimer row via a rhombus intermediate state and the other across the surface dimer row via a rotated rhombus intermediate state. These two-stage double piecewise rotation processes play a key role in lowering the kinetic barrier by establishing and maintaining energetically favorable bonding between adatoms and substrate atoms. These results provide an excellent account for experimental observations and elucidate their underlying atomistic origin that may offer useful insights for other surface reaction processes. DOI: 10.1103/PhysRevLett.97.046103 PACS numbers: 68.65.k, 68.35.Fx, 68.43.Bc, 81.07.b The understanding of kinetics and energetics of adatoms and small clusters that diffuse over the Si(001) surface has broad implications for the development of microscopic models for epitaxial growth [1,2] and for the fabrication of nanostructures such as quantum dots and nanowires [3]. In molecular-beam epitaxy, group-V elements (e.g., Sb and Bi) not only play a crucial role in heteroepitaxial crystal growth but also can self-assemble into ordered long ad- dimer lines [4 –7]. A key issue for the understanding of these phenomena is a quantitative description of the initial stage dissociation and diffusion of the precursor states towards stable ad-dimer configurations. Scanning tunnel- ing microscopy (STM) measurements reveal predominant Sb 4 [8 –10] and Bi 4 [11] tetramer precursor states on Si(001). Similar behavior was observed for Sb adsorbed on a Ge(001) surface [12]. Experimental evidence suggests splitting and rotation of the adsorbed Sb and Bi tetramers on Si(001) [9,13]. On the theoretical side, previous work has studied the rotation of individual group-V ad-dimers on Si(001) [14,15]. However, the more complicated and im- portant initial stage conversion processes of the experimen- tally observed precursor tetramer states to ad-dimer states have yet to be explored. In this Letter, we report on a detailed study of the energetics and kinetics of the dissociation processes of Sb and Bi precursor tetramer states on Si(001) using ab initio total-energy calculations. We reveal a novel two-stage dissociation mechanism involving double piece- wise rotation (DPR) processes that lead to lower kinetic barriers compared to those of direct dissociation pathways. We show that the ball-shaped precursor tetramer cluster from the gas phase adopts two primary adsorption configu- rations: one perpendicular to (T A ) and the other along (T B ) the substrate dimer row (see Fig. 1). The T A configuration first converts to an intermediate rhombus tetramer perpen- dicular to the substrate dimer row through a piecewise rotation process and then dissociates into ad-dimer states through a second piecewise rotation process. Meanwhile, the T B configuration can convert into an intermediate rhombus tetramer along the substrate dimer row with a small energy barrier and then further dissociate into the ad- dimer states through a similar double piecewise rotation process. At room temperature or above, configuration T B is expected to quickly convert into the rhombus configuration upon adsorption and, thus, elude experimental detection. FIG. 1 (color online). Optimized ad-dimer and tetramer con- figurations and their energies (in eV) defined in the text for Bi=Sb on Si(001). Large (red) and small (gray) circles represent the adatoms and the Si atoms in the top two layers of the substrate, respectively. Lines are drawn to indicate the electronic bonds. PRL 97, 046103 (2006) PHYSICAL REVIEW LETTERS week ending 28 JULY 2006 0031-9007= 06=97(4)=046103(4) 046103-1  2006 The American Physical Society