Role of surface strain in the subsurface migration of adsorbates on silicon

Juan Carlos F. Rodríguez-Reyes and Andrew V. Teplyakov

Phys. Rev. B 78, 165314 – Published 16 October 2008

Abstract

Modification of silicon surfaces through the insertion of atoms or even small molecular fragments of an adsorbate into a silicon-silicon bond can be affected tremendously by the effects of surface strain. This process takes place as either surface insertion or subsurface insertion, depending on whether the inserted species remains within the topmost layer or undergoes migration into subsurface layers, respectively. Using density-functional-theory cluster calculations, we show that insertion can be both thermodynamically and kinetically favorable if it takes place in such a way that surface strain is mitigated by neighboring surface sites. By considering the thermal decomposition of ammonia $({NH}_{3})$ adsorbed on a $Si (100) − 2 \times 1$ surface, we find that insertion mainly depends on the initial distribution of adsorbates and the orientation taken by inserted species with respect to neighboring structures along the surface. These factors seem to greatly affect the subsurface insertion, which can therefore be considered a long-range process. On the other hand, for surface-insertion processes the factors mentioned above are less influential, and insertion has more of a local character. Understanding the factors governing insertion mechanisms may lead to development of more approaches to surface functionalization, where the adsorbates decorating the surface can decompose in a controllable fashion.

Received 29 May 2008

DOI:https://doi.org/10.1103/PhysRevB.78.165314

Authors & Affiliations

Juan Carlos F. Rodríguez-Reyes and Andrew V. Teplyakov^*

Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, USA

^*Corresponding author. andrewt@udel.edu

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Vol. 78, Iss. 16 — 15 October 2008

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Images

Figure 1
Possible pathways corresponding to the thermal decomposition of ${NH}_{3}$ on the $Si (100) − 2 \times 1$ surface. Black: nitrogen; gray: silicon; white: hydrogen. Upon decomposition, structure A1 can undergo surface insertion (B1–B4) or subsurface insertion (S1–S4). For each structure, lateral and top views are included and, for clarification purposes, only silicon atoms representing the top two layers are shown in the lateral view. Surface models were simulated using the ${Si}_{9} H_{12}$ single-dimer cluster. In all cases, hydrogen terminations of the cluster are hidden. Notice the similarities between the tetracoordinated nitrogen species (B1, B3, S1, and S3) and their immediate successors (B2, B4, S2, and S4, respectively).Reuse & Permissions
Figure 2
Cluster models representing a $Si (100)$ surface.Reuse & Permissions
Figure 3
Effect of insertion on the mobile $H_{term}$ entities of the cluster model representing the surface. Gray: silicon; black: nitrogen; small white: hydrogen; large white: $H_{term}$ entities. For structures A1, B4, and S2 simulated using a single-dimer cluster model, the angles of these entities with respect to the surface normal are shown. Large arrows for structures B4 and S2 indicate the direction of strain, which opposes the most dramatic displacement of atoms on the surface. The replacement of $H_{term}$ entities by a neighboring dimer results in two possibilities of insertion, indicated as inward and outward. The stability of a structure varies depending on whether or not strain can be simulated by a neighboring dimer. Geometries were optimized at the B3LYP/ $6 − 31 + G (d)$ level of theory. Energies shown correspond to single-point calculations at the B3LYP/ $6 − 311 + G (d, p)$ level of theory. For clarification purposes, only silicon atoms representing the top two layers are shown for the single-dimer cluster models.Reuse & Permissions
Figure 4
Pathways of decomposition for an alternate structure A1-A1. Gray: silicon; black: nitrogen; white: hydrogen. The surface is represented by a ${Si}_{15} H_{16}$ double-dimer cluster model. Top views are shown for all structures, with H terminations of the cluster model hidden to facilitate visualization. For simplification purposes, tetracoordinated N structures B1, B3, S1, and S3 are not shown (those are similar to B2, B4, S2, and S4, respectively). Structures B4 and S2 feature three different orientations. Geometries were optimized at the B3LYP/ $6 − 31 + G (d)$ level of theory.Reuse & Permissions
Figure 5
Pathways of decomposition for an aligned structure A1-A1. Gray: silicon; black: nitrogen; white: hydrogen. The surface is represented by a ${Si}_{15} H_{16}$ double-dimer cluster model. Top views are shown for all structures, with H terminations of the cluster model hidden to facilitate visualization. For simplification purposes, tetracoordinated N structures B1, B3, S1, and S3 are not shown (those are similar to B2, B4, S2, and S4, respectively). Structures B4 and S2 feature three different orientations. Geometries were optimized at the B3LYP/ $6 − 31 + G (d)$ level of theory.Reuse & Permissions
Figure 6
Sequential model of insertion for an alternate distribution of adsorbates. The insertion process is simulated to take place once the neighboring dimer features an ${(Si)}_{2} NH$ inserted structure (B2 or S2 for surface or subsurface insertion, respectively). Gray: silicon; black: nitrogen; white: hydrogen. Although for subsurface insertion there are three possible structures, as shown in Fig. 4, only the one corresponding to the inward-inward orientation is displayed for clarification purposes. All the possibilities for both alternate and aligned distributions are discussed in the text.Reuse & Permissions

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